Factors associated with spontaneous vaginal birth in nulliparous women at term: a systematic review

doi:10.21203/rs.3.rs-1535021/v1

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Factors associated with spontaneous vaginal birth in nulliparous women at term: a systematic review

https://doi.org/10.21203/rs.3.rs-1535021/v1

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Objective: The aim of this systematic review was to identify the factors associated with spontaneous vaginal birth at term, in nulliparous women with singleton pregnancy.

Data sources: Nine databases were searched PubMed, MEDLINE Complete (EBSCO), Scopus, CINAHL Complete (EBSCO), Embase (Elsevier), Maternity and Infant Care (Ovid), Emcare (Ovid), Cochrane Central Register of Controlled Trials (CENTRAL which includes ClinicalTrials.gov) with dates extending from inception to 16 July 2021

Methods of Study Selection: Quantitative studies of all designs, published in English, of nulliparous women with a singleton pregnancy and cephalic presentation, who experienced a spontaneous vaginal birth (SVB) at term were included. Covidence was used to manage citation screening and full text review. Two reviewers undertook quality appraisal and RCTs with high risk of bias (ROB 2.0) and other designs (QATSDD) scoring ≤ 50% were excluded.

Tabulation, Integration, and Results: Data was abstracted from 78 studies (31 RCTs, 33 cohort, 9 cross-sectional, 4 prevalence and 2 case control) and factors associated with SVB were categorised as maternal, clinical care and fetal to synthesise findings. There was strong evidence to support interventions to address maternal BMI, birth intentions and fear of childbirth as well as childbirth education, breathing and exercise interventions in the antenatal period and midwifery models of care to increase SVB in nulliparas. Findings did not support routine induction as a means to promote SVB, but the relationship between maternal pelvic anatomy and fetal positioning and strategies to optimise labour progression emerged as promising areas for further research.

Conclusion: How women give birth the first time is important, and this review highlights key domains where evidence synthesis can guide improvements in care and direct future research.

Spontaneous vaginal birth

systematic review

caesarean section

nulliparous

How women give birth is important. In the past two decades, caesarean births (CB) have doubled¹ while instrumental (assisted and operative) vaginal births for first-time mothers have also steadily risen in high-resource contexts, leading to a decline in non-instrumental, spontaneous vaginal births (SVB)². No concomitant improvements have been afforded in maternal and neonatal morbidity resulting from these trends in mode of birth³. Conversely, rising caesarean birth rates in industrialised countries have been associated with increased risks of uterine rupture, abnormal placentation, and stillbirth⁴.

International recognition of the “caesarean epidemic” was highlighted in a 2018 Lancet series⁵. In response, large scale quality improvement projects in the United States (US) have reduced the primary caesarean birth rate among collaborating hospitals, from 29.3% in 2015 to 25.0% in 2017⁶, with concurrent improvements in neonatal morbidity⁶. However, the health care system, the context of birth, and consumer expectations in the US are manifestly different to those of other national contexts.

While a focus on reducing caesarean birth is important, given the maternal and neonatal risks of assisted vaginal birth⁷, actively promoting spontaneous, non-instrumental vaginal birth is ideal. This is particularly important in nulliparous women, due to the sequalae of a non SVB for births in future pregnancies. Vacuum and forceps assisted vaginal birth (here forth known as assisted vaginal birth or AVB) are both associated with an increased risk of injury to mother (including perineal trauma and episiotomy) and baby (including shoulder dystocia and intracranial haemorrhage)⁸. There is also high risk of psychological complications for the mother, with an association between a previous subjective negative birth experience and fear of childbirth which is greater than the association between previous obstetric complications and fear of childbirth⁹. Optimising opportunity for nulliparous women to have a SVB is vital; yet this has not been the focus of large-scale reform activities. Interdisciplinary efforts to specifically address the falling rates of SVB for nulliparous women are needed.

The aim of this systematic review was to identify the factors associated with spontaneous vaginal birth at term, in nulliparous, women with a singleton pregnancy. Exposures or interventions of interest included those related to safe and modifiable opportunities to attain a primary spontaneous vaginal birth, such as clinical practices, models of care and women’s own intentions and attributes.

Sources

Before commencing, a preliminary search of PROSPERO, MEDLINE, the Cochrane Database of Systematic Reviews and JBI Evidence Synthesis was conducted and failed to identify any current or preceding systematic reviews on the topic. A protocol detailing the background, methods, search strategies, data collection and analysis was developed and submitted to the PROSPERO registry (awaiting registration). The PRISMA 2020 Statement and Explanation and Elaboration document was used to guide review reporting¹⁰.

Nine databases were searched, comprising PubMed, MEDLINE Complete (EBSCO), Scopus, CINAHL Complete (EBSCO), Embase (Elsevier), Maternity and Infant Care (Ovid), Emcare (Ovid), Cochrane Central Register of Controlled Trials (CENTRAL which includes ClinicalTrials.gov) with dates extending from relevant database inception to 16 July 2021. References of included studies and prior systematic reviews were hand searched to locate additional studies. Grey literature was not searched. An expert medical librarian informed the search strategy design.

Initial scoping of search terms was undertaken by two authors (blinded for review) with the final list of search terms for inclusion determined after assessing the first 20 titles in PubMed and CINHAL for relevance and to identify other relevant keywords. A two-level search strategy was employed, using multiple terms related to the population (nulliparous women) and outcome of interest (spontaneous vaginal birth). No limitations (i.e., date, language, intervention type) were applied to the initial searches. Complete search strategies and inclusion criteria can be found in Appendix One, available online at (publisher to insert link).

Study Selection

Peer reviewed studies, published in scholarly journals, where participants were nulliparous women with a singleton pregnancy and cephalic presentation, who experienced a spontaneous vaginal birth at term (37–42 weeks) were included. A range of study designs was considered, including primary studies evaluating the effectiveness or efficacy of interventions relating to the outcome of a spontaneous vaginal birth, as well as epidemiological or observational studies used to investigate aetiology and/or factors which predict spontaneous vaginal birth, including prospective and retrospective cohort studies, case-control studies, and cross-sectional studies.

Qualitative studies and reports describing interventions (e.g., study protocols, guidelines) with no outcome data reported, were excluded. Studies that did not differentiate between instrumental and non-instrumental (i.e., spontaneous) vaginal birth, and studies that did not differentiate between nulliparous and multiparous women were excluded. As were studies in languages other than English.

Search results from different sources were merged and duplicates removed using EndNote X9 reference management software. The remainder of studies were screened by title and abstract, followed by full-text screening to remove irrelevant studies. All authors were involved in screening, with each article examined independently by two reviewers and discrepancies were resolved by discussion. Covidence (https://www.covidence.org/home) was used to manage citation screening and full text review.

Studies that met the inclusion criteria were assessed for methodological quality by two reviewers independently, prior to inclusion in the review. Randomised Controlled Trials (RCT) were assessed using the Cochrane risk-of-bias tool for randomised trials (ROB 2.0).¹¹ This is the preferentially recommended tool for RCT.^12,13 Studies deemed to have a ‘High’ overall risk-of-bias were excluded from the review. The Quality Assessment Tool for Studies with Diverse Designs (QATSDD)¹⁴ was used to assess other study types (i.e., quasi-experimental, case control, cohort, cross-sectional) and was modified to remove items relating solely to qualitative research. The QATSDD was developed specifically to enable quality comparisons between papers in reviews which include a variety of different study designs. Applying the QATSDD, each paper was given a score from 0 to 3 on 12 items, which was converted to a percentage with higher percentage denoting higher quality. Papers scoring ≤ 50% were excluded from the review. A data extraction template to identify pertinent information was developed, piloted and refined using a sample of two papers per study design. Qualtrics XM Survey Tool was used to record the appraisal of methodological quality and extract study data.

From 6438 studies a total of 78 studies met inclusion criteria and satisfied the quality appraisal (Fig. 1). Most studies were cohort (retrospective and prospective) design (n = 33), followed by randomised controlled trials (RCTs) (n = 31), cross-sectional (n = 9), prevalence (n = 4) and case control (n = 2). The majority of studies were conducted in high-resource settings (see Table 1) over a 30-year period (1991–2021).

Table 1

Summary of characteristics of the included studies
Author	Design	Year Country	Aim	Participants*	SVB rate	Quality appraisal^#
Randomised controlled trials
Beckmann et al⁹⁴	RCT	2019 Australia	To compare clinical outcomes following induction of labour (IOL) using a balloon catheter and going home, versus prostaglandin (PG) as an inpatient.	311	41.4 (I) 41.3 (C)	Low risk of bias
Bellussi et al⁶⁹	RCT	2018 Italy	To investigate the usefulness of visual biofeedback using transperineal ultrasound to improve coached pushing during the active second stage of labor in nulliparous women.	40	70% (Sonographic coaching) 65% (Traditional coaching)	Low risk of bias
Bick et al⁷⁵	RCT	2017 United Kingdom	In nulliparous women with epidural analgesia, does a policy of adopting an upright position throughout second-stage labour increase the incidence of SVB compared with a policy of adopting a lying-down position?	3236	Upright (35.2%) Lying down (41.1%)	Some concerns
Blanc et al⁸⁵	RCT	2021 France	To determine if the trial of prophylactic manual rotation at the early second stage of labour is associated with a decrease in operative deliveries (instrumental and/or CB	257	70.6% (I) 58.8% (C)	Low risk of bias
Bolanthakodi et al³⁸	RCT	2018 India	To test the hypothesis that prenatal yogic exercises are effective in alleviating labor pain and improving birth outcomes, by comparing pregnant women undertaking yoga with a control group.	200	68–82%	Some concerns
Cahill et al⁷⁷	RCT	2018 United States	To evaluate whether immediate or delayed pushing results in higher rates of spontaneous vaginal delivery and lower rates of maternal and neonatal morbidities	2414	85.9% (IP) 86.5% (DP)	Some concerns
Downe et al⁷⁴	RCT	2004 United Kingdom	To determine whether the rate of instrumental birth in nulliparous women using epidural analgesia is affected by maternal position in the passive second stage of labour	107	67.3% (Lateral) 48.3% (Sitting)	Some concerns
Fitzpatrick et al⁷⁸	RCT	2002 Ireland	To assess the effects of delayed vs immediate pushing in second stage of labour with epidural analgesia on delivery outcome, postpartum faecal continence and postpartum anal sphincter and pudendal nerve function.	178	IP (56%) DP (52%)	Some concerns
Fraser et al⁸⁰	RCT	2000 Canada	To determine whether a policy of delayed pushing for nulliparous women with epidural analgesia would reduce the risk of “difficult delivery” with respect to early pushing. Secondary objectives were to assess the effects of delayed pushing on spontaneous vaginal delivery, on indicators of maternal, fetal, and neonatal morbidity, and on women’s experience of delivery.	1862	57.5% vs 52.7%	Some concerns
Grobman et al⁵¹	RCT	2018 United States	The ARRIVE trial was designed to test the hypothesis that elective induction of labor at 39 weeks would result in a lower risk of a composite outcome of perinatal death or severe neonatal complications than expectant management among low-risk nulliparous women.	6106	74.1% (I) 69.3% (C)	Some concerns
Haakstad et al⁴⁰	RCT	2020 Norway	To investigate the sole effect of supervised group exercise, including pelvic floor muscle training on course of labour and mode of delivery.	105	62–75%	Some concerns
Hodnett et al⁵⁷	RCT	2008 Canada	To determine if a complex nursing and midwifery intervention in hospital labour assessment units would increase the likelihood of spontaneous vaginal birth and improve other maternal and neonatal outcomes.	4996	64% (I) 61.3% (C)	Some concerns
Janssen et al⁵⁶	RCT	2017 Canada	To determine if maternal characteristics measurable upon admission to hospital predict caesarean among low-risk spontaneously labouring nulliparous women	1459	75.6%	Some concerns
Karkada et al³⁶	RCT	2017 India	To examine the predictive factors for spontaneous onset of labor among women who have practiced antepartum breathing exercises.	280	Not reported	Some concerns
Kennell et al⁵³	RCT	1991 United States	In a US hospital with modern obstetric practices, 412 healthy nulliparous women in labor were randomly assigned to a supported group (n = 212) that received the continuous support of a doula or an observed group (n = 200) that was monitored by an inconspicuous observer	412	13.1% − 37.4%	Some concerns
Le Ray et al⁸¹	RCT	2009 Canada	To assess the influence of the duration of active second-stage labor on maternal and neonatal outcomes.	1,862	55.1%	Some concerns
MacArthur et al⁷¹	RCT	2001 United Kingdom	To compare low-dose combined spinal epidural and low-dose infusion (mobile) techniques with traditional epidural technique	1054	35.1% (traditional dose) 42.7% (low dose)	Low risk of bias
McCullough et al⁴²	RCT	2017 United Kingdom	To investigate the effects of antenatal reflexology on labour outcomes	61	62.5%	Low risk of bias
McLachlan et al⁴⁸	RCT	2012 Australia	To determine whether primary midwife care (caseload midwifery) decreases the caesarean section rate compared with standard maternity care.	2314	51.8% (I) 41.5% (C)	Some concerns
O'Sullivan et al⁵⁹	RCT	2009 United Kingdom	To investigate the effect of feeding (light diet) during labour on obstetric and neonatal outcomes, compared with water and/or ice chips only.	2426	44%	Low risk of bias
Plunket et al⁶⁷	RCT	2003 United States	To determine if waiting for a strong urge to push in nulliparas with continuous low-concentration epidural analgesia shortens the pushing duration in the second stage	202	Urge (70%) No urge (69%)	Some concerns
Rani et al⁶⁴	RCT	2015 India	To study the role of partogram in the management of labouring mothers with high-risk pregnancies at a tertiary care centre.	400	66% (No partogram) 71.5% (Partogram)	Low risk of bias
Rasouli et al³⁷	RCT	2016 Iran	To determine the effectiveness of motivational interviewing on women's participation in childbirth classes and their subsequent natural vaginal delivery	89	68.4% (I) 48.1% (C)	Low risk of bias
Reynolds et al⁷²	RCT	2003 United Kingdom	To investigate whether using low dose epidural infusion improves the normal delivery rate, outcome of labour was studied in women with singleton vertex presentations randomised to receive either 0.0625% bupivacaine opioid, or plain bupivacaine 0.125% for labour.	422	32% (plain bupivacaine) 40% (Bupivacaine + opioid)	Low risk of bias
Rouhe et al⁴³	RCT	2013 Finland	To compare the numbers of vaginal deliveries and delivery satisfaction among women with fear of childbirth randomised to either psychoeducation or conventional surveillance during pregnancy	371	65.6% (I) 47.2% (C)	Low risk of bias
Saad et al⁶⁸	RCT	2021 Egypt	The determine the proper timing of pushing during the second stage of labour in patients with OP position and using epidural anaesthesia and to assess the effect of maternal bearing down timing either early or late on foetal wellbeing as well as on the incidence of instrumental delivery and rate of caesarean section.	184	IP (80.4%) DP (60.9%)	Low risk of bias
Seyedi et al⁶¹	RCT	2020 Iran	To determine the effect of oral sodium bicarbonate solution on delivery out-come of primiparous women with labor stagnation.	142	53.7% (I) 46.3% (C)	Low risk of bias
Shen et al⁹⁸	RCT	2017 United States	To evaluate whether maintaining a motor sparing epidural analgesia infusion affects the duration of the second stage of labor in nulliparous parturients compared with a placebo control	400	96.5% (Epidural) 99% (Saline only)	Low risk of bias
Stoddart et al⁹⁹	RCT	1994 United Kingdom	The aim of this study was to determine the effect on the instrumental delivery rate of two different concentrations of bupivacaine combined with fentanyl in epidural infusions during labour.	40	75% (no epidural)	Some concerns
Wiberg-Itzel et al⁶²	RCT	2018 Sweden	Labor dystocia is an intransigent, high-profile issue in obstetric care. Amniotic fluid lactate (AFL) reflects the uterine metabolic status. High levels associate with subsequent need for operative intervention due to dystocia. In sports medicine, it is known that lactic acid can affect muscular performance and can be decreased by bicarbonate given orally before physical activity	143	84.4% (I) 67.7 (C)	Low risk of bias
Case control studies
Darsareh et al³³	Case control study	2019 Iran	The purpose of study was to evaluate the effect of a campaign based on social marketing to promote normal childbirth	350	74%	88%
Kirici et al³⁹	Case control study	2021 Turkey	This study aimed to evaluate the effects of Pilates workouts on labor and newborn outcomes.	Intervention (n = 320) Control (n = 220)	92.3% (I) 58.8% (C)	64%
Cohort studies
Ahmad et al⁸²	Cohort study	2014 United Kingdom	To determine the association between fetal position at onset of labor and mode of delivery, specifically left occipito-anterior (LOA) fetal position and SVB	1250	52.9%	83%
Boujenah et al²²	Cohort study	2020 France	To study the mode of delivery in a well selected cohort of short nulliparous women (defined as 150cm, − 2 SD of the average population size over the same period:2010–2018).	178	35.4%	89%
Brown et al⁹¹	Cohort study	2016 Australia	The unexpected loss of the CFM system at a tertiary unit provided a unique opportunity to evaluate outcomes and staff attitudes toward CFM	1,600	42.1% (CFM) 48.4% (No CFM)	72%
Carlhall et al²⁵	Cohort study	2020 Sweden	To evaluate whether the duration of labor was associated with maternal BMI in primiparous women with induction of labor	15,259	BMI ≥ 40 (72%) 69.9% (average)	72%
Dalbye et al²⁴	Cohort study	2021 Norway	This study investigates associations between maternal body mass index (BMI) early in pregnancy and obstetric interventions, maternal and neonatal outcomes.	7189	Underweight SVB 78.5% Obesity II-III SVB 71.3%	81%
Dietz et al⁸⁸	Cohort study	2006 Australia	To identify potential predictors for emergency operative delivery.	202	61%	61%
Dunn et al¹⁹	Cohort study	2017 Australia	To determine if Advanced Maternal Age is associated with emergency caesarean birth (CB) following IOL.	7,459	60.7% (Maternal age < 38 years) 64.1% (Maternal age ≥ 38 years)	58%
Edmonds et al¹⁶	Cohort study	2013 United States	To assess the extent to which primary, unscheduled caesarean deliveries and their indications vary by race/ethnicity in one academic medical centre	4,483	74.1%	72%
Essex et al¹⁸	Cohort study	2013 United Kingdom	To explore the maternal demographic factors associated with operative births (instrumental vaginal births or caesarean birth), after adjustment for health, interpersonal, pregnancy, labour and infant covariates	7,432	57%	86%
Fitzwater et al¹⁰⁰	Cohort study	2015 United States	To evaluate changes over the past decade in the mode of delivery and second-stage duration in nulliparous women.	1,023 (historical) 1,476 (contem)	77% (historical) 84% (contemporary)	81%
Frey et al⁷⁶	Cohort study	2012 United States	To estimate maternal, neonatal, and labor outcomes associated with delayed pushing.	1,972	DP (72.9%) IP (77.4%)	58%
Ghi et al⁸⁴	Cohort study	2016 Italy	To determine whether the subpubic arch angle (SPA) measured by three-dimensional ultrasound is associated with the fetal occiput position at delivery and the mode of delivery.	368	66.1% (OA) 13.5% (OP)	61%
Gidaszweski⁴⁵	Cohort study	2019 Australia	To evaluate the effect of a caseload midwifery program (CMP) on birth outcomes and rates of perinatal interventions at a metropolitan tertiary hospital in Australia, compared with standard midwifery-led care (SMC)	Matched sample, 500 in each group.	69% (I) 50% (C)	75%
Gupta et al²⁰	Cohort study	2008 United Kingdom	To quantify the age-related risk of adverse obstetric outcome in primigravid women less than 20 years of age.	66,271 between 1990–1999. Compared < 20 years with 20-<35 years.	69.4% <20 years 51.8% 20–35 years	55%
Henderson et al⁶⁵	Cohort study	2014 Italy	To describe maternal characteristics, intrapartum events, interventions, maternal and neonatal outcomes for all women who used a birthing pool during labour who either had a waterbirth or left the pool and had a landbirth, and for the subgroup of women who had a waterbirth in 19 obstetric units, and (b) to compare maternal characteristics, intrapartum events, interventions, and maternal and neonatal outcomes for women who used a birthing pool with a control group of women who did not use a birthing pool	2,505	95.6%	67%
Ibison et al¹⁷	Cohort study	2005 United Kingdom	To investigate the relationship between 'ethnicity' and mode of delivery in first-time mothers	27, 667	78.7% (Total) 71.8% (African) 80.2% (Pakastani)	53%
Johnson et al⁴⁶	Cohort study	2005 Australia	Maternal and infant clinical outcomes were compared for low-risk mothers receiving a partnership caseload model of midwifery care, known as Primary Health Midwifery Care (PHMC), and standard hospital care (SHC)	408 (CoC group) 349 (Control)	74.8% (I) 71.9% (C)	69%
Kehl et al⁵⁴	Cohort study	2019 Germany	To evaluate the efficacy of induction of labour in obese women using sequential double-balloon catheter and oral misoprostol in comparison with oral misoprostol alone.	400	65.4% (combined IOL) 53.3 (Miso only IOL)	55%
Kringeland et al³⁴	Cohort study	2010 Norway	To estimate to which extent women who expressed a preference for natural birth actually were delivered vaginally without interventions, and second, to estimate the influence that emotions and maternal background factors have on the mode of delivery	39,475	29.3%	75%
Lipschuetz et al¹⁰¹	Cohort study	2015 Israel	We examined associations between neonatal head circumference (HC) and delivery mode and risk of perinatal complications as compared to high BW	6343	≥ 95th centile HC 38% < 95th centile HC 63.4%	67%
Lundgren et al⁵⁵	Cohort study	2013 Sweden	To explore the frequency of women seeking care during the latent phase of labour at two delivery wards. Also, to investigate interventions during labour and birth outcomes in relation to parity and remaining in hospital or not during the latent phase.	5797	83.4% (DC home) 73.6% (remained in hospital)	67%
Masri et al⁷⁹	Cohort study	2021 Germany	To evaluate the effect of combining the technique of early pushing and extended second stage on the mode of delivery, as well as adverse maternal and neonatal outcomes.	5971	76% − 92.9%	58%
Nielsen et al⁴¹	Cohort study	2017 Denmark	To examine the association between maternal leisure time physical activity and mode of delivery.	2,435	76%	58%
Nelson et al⁸⁷	Cohort study	2021 Australia	To examine if there is any association between sonographic fetal HC, obstetric anal sphincter injury (OASIS) and mode of delivery.	667	73%	65%
Offerhaus et al³²	Cohort study	2015 The Netherlands	To study whether an increase in intrapartum referrals in primary midwife-led care births in the Netherlands is accompanied by an increase in caesarean sections.	789,795	84% (I) 77% (C)	72%
Oliphant et al²⁸	Cohort study	2014 United States	The objective of the study was to define maternal tissue adaptations in pregnancy associated with uncomplicated spontaneous vaginal delivery using anatomical and biological outcomes	150	59%	61%
Ponkey et al⁸³	Cohort study	2003 United States	To evaluate the obstetric outcomes associated with persistent occiput posterior position of the fetal head in term laboring patients	2997	26% (OP) – 74% (OA)	58%
Turner et al⁷⁰	Cohort study	2020 Australia	This study aimed to assess the impact of epidural anaesthesia on maternal and neonatal adverse outcomes when the second stage of labour was prolonged	48,342	20.7% (Epidural PSSL) – 91.2% (no epidural, no PSSL)	64%
Valente et al²³	Cohort study	2020 Portugal	To describe the effect of obesity on the duration of labour, specifically latent and active phases of labour, and on mode of delivery, among nulliparous women with epidural analgesia	150	28.9% Normal Weight 32.9% Obese	64%
van de Pol et al²⁷	Cohort study	2006 The Netherlands	To examine several factors which could be associated with the risk for instrumental and surgical delivery. In addition to biomedical factors we included psychosocial factors such as depressive symptoms, quality of the relationship of the woman with her partner, personality, lifestyle and educational level.	354	71.7%	83%
Van Veelen et al²⁹	Cohort study	2015 The Netherlands	To determine the association between levator hiatal dimensions, measured using transperineal ultrasound, in women during their first pregnancy and the subsequent mode of delivery, stratified by the indication for intervention.	280	70.6%	55%
Tracy et al⁹⁵	Cohort study	2007 Australia	We aimed to determine the association between interventions introduced during labour with interventions in the birth process amongst women of low medical risk	145,211	56.8%	78%
Wong et al⁴⁷	Cohort study	2015 Australia	To compare outcomes for a cohort of low-risk primiparous women who accessed a midwifery continuity model of care with those who received standard public care in the same tertiary hospital	1646	57.7% (CoC) 48.9% (Standard)	75%
Cross sectional studies & prevalence studies
Afshar et al³⁵	Cross sectional study	2017 United States	To determine whether the mode of delivery was different between women who attended childbirth education (CBE) class, had a birth plan, or both compared with those who did not attend CBE class or have a birth plan	9168	66–71%	78%
Bugg et al¹⁰²	Prevalence study	2006 United Kingdom	To highlight the differences in mode of delivery between women augmented with intravenous oxytocin because of failure to progress in labour with those who labour without the need for augmentation.	3842	51.1% (Augmented) 76.6% (Not augmented)	78%
Dadipoor³⁰	Cross sectional study	2017 Iran	To anticipate the type of childbirth according to the health belief model	222	66.4%	72%
Dahlen et al¹⁵	Prevalence study	2013 Australia	To compare the risk profile, rates of obstetric intervention and selected maternal and perinatal outcomes for low-risk women born in Australia compared to those born overseas	124,431	51.4% (Indian born) 75.2% (Lebanese born)	83%
Ebrahimzadeh et al⁵⁸	Cross sectional study	2012 Iran	To evaluate the correlation between maternal fatigue and uterine contraction pattern at the beginning of the active phase of labour.	100	36.5% (severe fatigue) 73.9 (mild fatigue)	72%
Ghaffari et al³¹	Cross sectional study	2017 Iran	The objective was to determine which theory of planned behavior (TPB) constructs predict intentions and delivery mode among pregnant women	104	71.15% SVB rate	64%
Ghotbi et al¹⁰³	Cross sectional study	2012 Iran	The aim of this study was to determine the rate of CB on mother's request (CDMR) and to determine maternal attitude and knowledge about various modes of delivery in private and public (university) hospitals in Tehran	600	16.5%	92%
Henderson et al²¹	Cross sectional study	2016 United Kingdom	To see whether, compared with women aged 21 or more, women aged 20 years or younger worried more about labor and birth, and had poorer maternal outcomes. Another objective was to investigate the extent to which worries about labor and birth mediated the associations between young age and outcomes.	5332	52.4%	67%
Nyman et al⁹⁰	Prevalence study	2017 Sweden	To describe the use of interventions during labour and birth in healthy women at term with spontaneous onset of labour, before and after initiation of an Action Research project	903	2009 (85.6%) 2012 (87.4%).	78%
Paterson et al¹⁰⁴	Prevalence study	1992 United Kingdom	To define the contemporary characteristics of the second stage of labour in one Health Region	10,932	57.8% (Epidural) 86.7% (No epidural)	56%
Prosser et al²⁶	Cross sectional study	2018 Australia	We examined the role of modifiable and non-modifiable factors in experiencing a normal birth using retrospective, self-reported data	2640	53.7%	81%
Schytt et al⁷³	Cross sectional study	2008 United Kingdom Sweden	Data were examined from two independent questionnaire-based longitudinal studies, 'KUB' in Sweden (N = 3113) and 'Greater Expectations?' (N = 1439) in England, which had similar aims and methodology. Our joint data sets offered us the opportunity to look for similarities and differences between the samples that might contribute to women's experiences of childbirth and mode of birth, as well as psychosocial sequelae	4552	74.4% Sweden 54.7 England	72%
Symon et al⁴⁹	Cross sectional study	2007 United Kingdom	To compare outcomes for women intending to give birth in these different types of unit and whose self-rated pregnancy risk level was "none" or "low."	432	93.2% (MW led unit) 57.1% (Obs led unit)	64%
*Nulliparous women only ^#Cochrane risk-of-bias tool for randomised trials (ROB 2.0); Quality Assessment Tool for Studies with Diverse Designs all other study designs

*Databases and registered searched

Factors associated with SVB in nulliparous women at term were across the antenatal and intrapartum periods and included maternal, clinical care and fetal factors. Findings were categorised into meaningful constructs and ordered according to progression through the perinatal period (See Fig. 2).

MATERNAL

Maternal characteristics

Maternal demographic and physical characteristics have been linked to mode of birth and this was evident in 15 included studies. In a large Australian retrospective cohort study, country of birth was significantly associated with likelihood of SVB in nulliparous women. Frequency of SVB ranged from 51.4% for women born in India and the Philippines through to 75.2% for women born in Lebanon¹⁵.

Ethnicity has also been reported as influencing the outcome of SVB. A US study found Black and Asian women were less likely to birth spontaneously than Caucasian women and were more likely to experience operative birth due to fetal distress and other disorders diagnosed in the first-stage of labour compared with the second stage, when birthing within the US hospital system¹⁶. A large cohort study of low-risk nulliparous women in East London, found women from Africa, West Indies, Bangladesh, India and Pakistan had an increased likelihood of CB (OR: 2.8, 1.7, 2.1, 1.6 & 1.5 respectively) than Caucasian women after controlling for confounders¹⁷.

Conversely, in the Millennium Birth Cohort study among the nulliparous group (n = 7,432), black women were less likely to have an AVB, and women of mixed ethnicity were less likely to have an emergency CB. In the same study, language and occupational level were contributing factors as women in non-English-speaking households were less likely to have an AVB, than women in English speaking households¹⁸. While women from English-only speaking households and households of lower occupational level were at increased likelihood of caesarean section¹⁸ as were older women (> 35 years).

Advanced maternal age (≥ 38 years) was also associated with a decreased likelihood of SVB [aOR 1.15 (0.98–1.35) P = 0.080] for women following induction of labour in an Australian study, however this was not statistically significant¹⁹. In other studies, teenage women were more likely to have a SVB (OR = 2.1(2.0–2.3)) with a significantly lower incidence of AVB (OR = 0.5 (0.5–0.6)), and CB (OR = 0.4(0.4–0.5))^20,21.

In terms of physical characteristics, maternal height was the focus of a small cohort study on mode of birth. The study ascertained that for women of short stature (< 150cm), after adjustment for confounders, the increased risk of CB remained only for neonatal birth weights > 3500g (aOR4.3; 95%CI 1.96–10.2)²², compared to those < 3499g.

Maternal Body Mass Index (BMI) has been investigated in several cohort studies. A Portuguese study reported that obese women had less frequent SVB compared to women of normal weight (28.9 V 32.9% respectively) and took longer to establish active labour when birth was induced²³. Similarly, a Norwegian cohort study found a trend suggesting that with increasing BMI there was reduced SVB [Intrapartum CB in women of obesity class I and obesity classes II and III, with adjusted OR of 1.70 (95% CI 1.21–2.38) and 2.31 (95% CI 1.41–3.77), respectively]²⁴. Contrary to this, in a Swedish cohort of nulliparous women (n = 15 259) with induction of labour, there were no significant differences between obese and normal weight in terms of SVB (78.5% underweight – 72% BMI ≥ 40)²⁵.

A small cohort study in the US²⁸ investigated pelvic tissue adaptations in pregnancy and their potential association with SVB. They found that vaginal support changed significantly over trimesters with posterior vaginal and hiatal relaxation, vaginal lengthening, and increased levator hiatus area during strain. Women birthing spontaneously demonstrated significantly greater relaxation on third-trimester Pelvic Organ Prolapse Quantification for anterior, apical, and hiatal measures than those without uncomplicated SVB and higher first trimester vaginal elastase activity was highly associated with SVB (geometric mean activity 0.2890.830 U/mg vs -0.0290 ± 585 U/mg, P = 0.009)²⁸. A Dutch study also found an association in nulliparous women between smaller levator hiatal dimensions on pelvic floor contraction and associated assisted vaginal birth or CB due to ‘failure to progress’ (p = 0.005)²⁹.

An Australian cross-sectional study found conditions arising during pregnancy, such as gestational diabetes (M 0.68 CI [0.48.0.91]) and high blood pressure (0.39 CI [0.26–0.53]) reduced the likelihood of achieving a ‘normal birth’. The authors defined normal birth as spontaneous onset of labour, no regional anaesthesia, having an unassisted vaginal birth and no caesarean or episiotomy²⁶. However, the outcome variable of an unassisted vaginal birth was not statistically significantly different when measured independently for gestational diabetes (M 0.97 CI [0.90–1.01] or high blood pressure (M1.02 [0.99–1.06]²⁶.

Maternal mindset

Six studies described women’s intentions, preferences, and beliefs regarding mode of birth. A cross sectional study³⁰ explored the intentions of 222 Iranian women and found a significant correlation between perceived barriers to normal birth and the mode of birth. In this cohort of women, 33.6% intended a CB. The intended and actual place of birth were correlated over time in a large prevalence study of 95% of births in the Netherlands³². Although women’s intentions to birth at home decreased by 20% between 2000–2008, those who did birth at home had the highest rates of SVB (57.2%), when compared with those who planned a hospital birth (49.4%)³².

Another Iranian cross-sectional study also found significant correlations between the attitudes, subjective norms and perceived behavioural control women held toward SVB and their actual mode of birth³¹, with those intending an SVB, more likely to achieve one [Attitude toward SVB (p < 0.0001), Subjective norms (p < 0.05) and Perceived Behavioural Control (p < 0.001)] were significantly correlated. Follow-up with the women, found that rates of 71.15% (SVB) and 28.14% (CB) were achieved, particularly pronounced in Iran where maternal preference for caesarean birth is as high as 58%³¹. A large Norwegian cohort study found that a preference for natural birth was not associated with SVB, however in this study, only 29.3% of women birthed vaginally without any interventions³⁴.

Countries such as Iran, where very high operative birth rates persist, have reported success with public health campaigns targeting unhelpful constructs around normal birth³³. While the CB rate remained very high in this population (74.3% overall), a health belief model intervention (B-Butterfly campaign), which actively promoted normal childbirth through social media, saw a 35.6% SVB rate compared with 13.5% in the control group (p < 0.001, df = 1)³³. The predominant method of birth across both groups was CB caesarean (74.3% )³³.

In terms of psychosocial traits, no association was found between personality traits or depressive symptoms and mode of birth²⁷. A moderately sized cross-sectional study explored potential relationships between partner support and mode of birth and found that social support from a woman’s partner was not predictive of SVB²⁷.

Maternal preparation

Few (n = 9) studies identified a focus on mechanisms for maternal preparation during the antenatal period. Studies covered areas such as childbirth education programs, low-impact exercise programs, complimentary therapies, and interventions to address severe fear of childbirth. Antenatal classes through formalised childbirth education and an intentional plan for birth have been shown to increase likelihood of an SVB³⁵. An RCT conducted in India (n = 280), demonstrated that an intervention (a short video on LAMAZE breathing, one-to-one instruction and twice a day practice) resulted in women in the experimental group with spontaneous onset of labour having 1.93 odds [95% CI (1.302–2.862), p = .001] of SVB compared with controls. Also, women whose period of gestation was 39–40 weeks at the time of birth had 1.368 odds [95% CI (1.905–2.069), p = 0.013] of SVB than controls³⁶. In Iran, motivational interviewing, coupled with childbirth education, has also proved helpful in challenging constructs around normal birth, with up to 20% improvement in SVB for women receiving the antenatal intervention³⁷.

A Norwegian RCT also investigated the effect of antenatal exercise on mode of birth. Women randomised to the intervention group (exercise 60 min 2/week) had higher rates of SVB (85.7%) compared with controls (62.3%) (p = 0.051) although statistical significance was not reached given the small sample size⁴⁰. Similarly, a Danish study of moderate levels of exercise, in the 1–2 years before pregnancy and during each of the trimesters⁴¹ confirmed that women with low activity had a 28% increased odds of a more complicated birth (AVB or CB)⁴¹.

Low impact exercise has also been investigated. Another RCT in India found a significant improvement in SVB (p < 0.037) for women randomised to a 30-minute integrated yoga intervention practiced at home (fortnightly from 30 to 36 weeks and then weekly until birth)³⁸. Similarly, a Turkish study found a pregnancy modified Pilates program which involved Pilates 70–80 minutes per week for eight weeks resulted in a statistically significant (P < 0.001) increase in likelihood of SVB³⁹.

Complimentary therapies have been purported to effect mode of birth. One pilot study (not powered to detect differences), ascertained whether reflexology for persistent back and pelvic girdle pain effected mode of birth⁴². They reported no significant difference (p = 0.716) in mode of birth between the reflexology, sham (foot bath) or standard care groups⁴².

Screening for and addressing fear of childbirth is an emerging area of scholarship. In a Finish study, women with identified severe fear of childbirth were randomised to an antenatal psychoeducation intervention (psychoeducative group therapy, six sessions during pregnancy and one postnatally) or usual care⁴³. There was a significant increase in SVB in the intervention arm (63.4% versus 47.5%, p = 0.005) and fewer CBs (22.9% versus 32.5%, p = 0.05). Women in the intervention group more frequently reported a very positive experience of their childbirth⁴³.

CLINICAL CARE

Several studies (n = 23) investigated the impact of clinical care, including models of care, interventions in first and second stages of labour on mode of birth, methods of onset of labour, labour support, maternal wellbeing during labour, interventions for addressing dystocia and use of tools, such as partographs, to intervene in labour progress.

Models of care

Continuity of midwifery care, where a known midwife or small team of midwives, provide antenatal, intrapartum and postnatal care to women⁴⁴ increased SVB rates, when compared with a matched cohort receiving standard midwifery care (69% vs. 50%, OR = 1.79,95% CI = 1.38–2.32)⁴⁵. In an Australian matched cohort study, partnership, caseload midwifery care improved the likelihood of SVB (OR 1.15 95% CI 0.83–1.59) compared with standard hospital care, however this difference was not statistically significant⁴⁶. Similarly, in a convenience sample cohort in the Netherlands, women receiving caseload midwifery care had a higher SVB rate (84.0%) than those receiving standard care (77.0%)³². This finding was supported in a convenience sampled Australian cohort⁴⁷, where women who chose midwifery continuity of care and planned birth in a birth centre, had a higher likelihood of an SVB aOR (2.37 [1.85–3.05 95% CI]), and were less likely to request and receive epidural analgesia⁴⁷. A large RCT found this difference in SVB particularly significant in low-risk women (51.8% V 41.5%; RR 1.25; 95% CI 1.12–1.39; P < 0.001), when compared with standard public midwifery care⁴⁸.

The effect on outcomes by model of care and place of birth (midwife-led care v obstetric-led care) persisted in a sample restricted to women who self-identified as ‘no- or low-risk’ mothers⁴⁹. Those birthing in midwife-led units experienced shorter hospital stays, received less pharmacological analgesia, fewer interventions and were significantly more likely to have a SVB (93.2. V 57.1%; x² 36.69; p < 0.01)⁴⁹.

First stage of labour

The onset of labour, whether spontaneous or induced, has been the focus of much contemporary research. Studies have compared different approaches to induction of labour (IOL) and an option of routine IOL with regard to mode of birth outcomes. An Australian RCT found no difference in SVB outcomes between nulliparous women induced with a balloon catheter versus those induced with PGEs (41.4% V 41.3% respectively)⁵⁰. However, the ARRIVE Trial⁵¹ of labour induction versus expectant management in low-risk nulliparous women, found a decreased likelihood of SVB and increase in CB in the expectant management group (569 (18.6%) V 674 (22.2%) RR 0.84 (0.76–0.93) < 0.001). In an Australian prevalence study of women with low medical risk, rates of operative birth increase markedly - AVB (aOR 8.07 [8.58–9.46]) and emergency CB (aOR 52.18 (46.92–58.04)) – for women who were induced and had epidural analgesia⁵².

However, for women with BMI > 35, cervical ripening and IOL methods may have more discrete impact. In a German cohort⁵⁴, IOL with sequential use of double-balloon catheter and oral misoprostol was associated with significantly more SVB and fewer CB (38.6% versus 56.9%, p = 0.0039) compared to oral misoprostol alone for obese nulliparous women.

Following labour onset, progression through the first stage of labour and interventions to address dystocia have been hypothesised to contribute to mode of birth. Location of early labour has been associated with mode of birth. A large Swedish cohort study found that SVB occurred less frequently in women presenting in the latent phase of labour and remaining in hospital (73.6% versus 83.4% p = 0.007) compared to those who went or remained at home⁵⁵. Similarly, data from a clinical trial⁵⁶ revealed that women who presented early in the latent phase of labour, with mild and irregular uterine activity, who were older and shorter, and had a perception of labour lasting more than 24 hours, with a higher fetal station on admission, were less likely to birth spontaneously (the C-statistic for the predictive model was 0.71 (0.64–0.75) and the sensitivity and specificity of the model were 0.80 (95% CI 0.76–0.84) and 0.48 (95% CI 0.44–0.52), respectively⁵⁶.

Clinical trials aimed at addressing labour dystocia with augmentation were reported in the included studies. Augmentation with synthetic oxytocin due to slow or stalled labour progress was explored in a UK prevalence study⁶⁰. Only 51.1% of nulliparous women who received augmentation had a SVB compared with 76.6% of women who did not receive augmentation (RR 0.67, CI 0.63–0.71). In addition, more women whose labours were augmented had epidurals (62.0% versus 25.5%; RR 1.89, CI 1.62–2.21), became pyrexial (7.7% versus 2.5%; RR 3.14, CI 2.30–4.29), had a PPH > 1000 mls (1.5% versus 0.7%; RR 2.23, CI 1.14–4.37) and received blood transfusions (3.4% versus 1.8%; RR 1.90, CI 1.25–2.91) when compared with those who laboured without oxytocin⁶⁰.

Two subsequent RCTs have investigated the effect of oral sodium bicarbonate solution on mode of birth in women with labour dystocia^61,62. Both studies found a significant improvement in SVB groups where an oral dose of sodium bicarbonate was given when confirmed labour dystocia occurred (Intervention 53.7% v Control 46.3%; p 0.033⁶¹ and Intervention 84.4% V Control 67.7%, p < 0.01⁶²). This was effective in both induced⁶¹ and spontaneous labours⁶².

The use of the partograph has been suggested to enhance a normal birth. An RCT in India, randomised 400 high risk women (defined as those with pregnancy induced hypertension, gestational diabetes, cholestasis, history of a LSCS, hypothyroidism), to care according to the WHO partograph⁶³, or not. There was no statistically significant difference in SVB rates between the groups (71.5% partograph versus 68% no partograph, p = 0.584)⁶⁴.

In seminal work in the early 1990’s, continuous support in labour (Doula support) in addition to partner support had a positive effect on SVB in a US hospital⁵³. A more structured approach to ‘intentional nursing care’ during early labour admission found no statistically significant increase in the likelihood of SVB (OR 1.12, 95% CI 0.96 to 1.27)⁵⁷.

Maternal wellbeing, specifically maternal fatigue and adequate hydration and nutrition have been examined regarding effect on mode of birth. Self-measured via a Visual Analogue Scale, maternal fatigue was correlated with increased likelihood of disrupted contraction pattern and reduced likelihood for SVB in a small cross-sectional study of Iranian women (n = 100), where fatigue level was higher for CB (6.45 ± 1.7) compared to those with VB (5.84 ± 3.3)⁵⁸. Furthermore, a large RCT in the UK randomised nulliparous women to a light food diet in labour or water only. Key findings were that SVB rates were the same in both groups (44%; relative risk 0.99, 95% CI 0.90 to 1.08). No clinically important differences were found in the duration of labour (geometric mean: eating, 597 min v water, 612 min; ratio of geometric means 0.98, 95% CI 0.93 to 1.03), the CB rate (30% v 30%; relative risk 0.99, 0.87 to 1.12), or the incidence of vomiting (35% v 34%; relative risk 1.05, 0.9 to 1.2)⁵⁹.

Second stage of labour

Various studies examined components likely to affect the second stage of labour and mode of birth (n = 16) and focused predominantly on situations where women had concurrent epidural analgesia (n = 15).

Only one study addressed non-epidural pain relief investigating warm water immersion and water birth in relation to effect upon SVB. In a large Italian study, no difference in SVB was seen with women using warm water immersion, compared with those labouring on land only; however, within this cohort, SVB rates were very high (> 90%)⁶⁵.

For women with epidural analgesia the risk of AVB is much higher⁶⁶. Strategies trialled to reduce this encompass dosage, fetal and maternal positioning, timing of pushing and obstetric interventions. Over the past 30 years changes to dosage regimes in epidural analgesia have been investigated regarding their effect on mode of birth. Large cohort studies have demonstrated that nulliparous women with epidural analgesia have significantly lower odds of birthing spontaneously⁷⁰. Low-dose epidurals (sometimes known as walking epidurals) have been found to be superior in terms of opportunity for SVB (dosage 0.1% bupivacaine with 0.1% fentanyl infusing at 10 ml/hr)⁷¹. A UK RCT (n = 602) investigated whether a lower dose enhanced opportunity for SVB (0.0625% bupivacaine with either fentanyl 2.5 lg/mL or Sufentanil 0.25 lg/mL v 0.125%); no statistically significant improvement was evident for nulliparous women⁷². However improved maternal satisfaction and mobility was afforded with the lower dose and the authors recommend this as contemporary best practice⁷².

Furthermore, the analgesic effectiveness of epidurals, as perceived by women, has been correlated with mode of birth⁷³. Differences between two matched samples (in Sweden and England), found the likelihood of an unassisted vaginal birth were particularly marked for women who had an epidural and reported that it was effective (x2 = 51.72, 1 df, p 0.001). Less effective epidurals resulted in increased frequency of unassisted vaginal birth, although this association was not statistically significant (69.8% V 56.4%)⁷³.

A UK RCT⁷⁴ (n = 107) compared two maternal positions during the passive second stage of labour, on the rate of instrumental vaginal birth in nulliparous women who had chosen to use epidural analgesia. Lateral positioning was associated with SVB (aOR 2.3), when compared with sitting during passive second stage. The UK RCT known as the BUMPES trial, compared maternal positioning during second stage labour when epidural analgesia was in situ, for nulliparous women⁷⁵. Following randomisation of 3236 women, a lying down position afforded a significant increase in SVB, when compared with a semi-recumbent, upright position (41.1% V 35.2% adjusted RR 0.86 (0.76–0.94)⁷⁵.

The timing of pushing for women with epidural analgesia has also been the focus of several studies. An RCT with nulliparous women (n = 202) compared standardised epidural analgesia (0.0625% bupivacaine with fentanyl 2 µg/mL), between pushing immediately on complete cervical dilatation vs waiting for a strong urge to push⁶⁷. They found delayed pushing until a strong urge is present had no effect on the duration of second stage of labour or likelihood of an SVB (70% v 69% respectively)⁶⁷. In a more recent RCT (n = 184), early pushing (within 1 hour of confirmed full dilatation) was compared with delayed pushing in women with epidural analgesia, and a fetus in the OP position⁶⁸. The rate of SVB was significantly higher in the group who commenced pushing early (80.4 vs. 60.9%, p < 0.004). They also found an increase in perineal tears in the early pushing group⁶⁸. An Italian trial examined sonographically coached pushing in a small (n = 40) group of nulliparous women, with and without regional analgesia⁶⁹. They found an overall reduction of 15 minutes in the sonographically coached group, but no difference in mode of birth ⁶⁹.

A large retrospective study found no advantage in delaying active pushing in second stage for nulliparous women (DP 210 (72.9%) V IP 104 (77.4%) RR 0.94 (0.87–1.01) p 0.09)⁷⁶. A subsequent RCT (n = 2414) compared immediate versus delayed (60 minutes) active pushing and found no significant difference in the rates of SVB (85.9% vs 86.5%, respectively). Interestingly, mean duration of second stage was shorter in the immediate pushing group and PPH higher in the delayed pushing group, leading to early cessation of the trial, as interim analysis demonstrated futility in being able to demonstrate a difference in the primary outcome (102.4 vs 134.2 minutes, respectively; mean difference, − 31.8 minutes [95% CI, − 36.7 to − 26.9], P < .001; PPH (2.3% vs 4.0%; between-group difference,−1.7% [95% CI, − 3.1% to − 0.4%], P = .03)⁷⁷. An Irish RCT found similar rates of AVB between cohorts of immediate and delayed pushing during the second stage of labour⁷⁸, with delayed pushing leading to a significantly longer duration of labour (427 vs 480minutes, p = 0.005). Whereas, a large German cohort study (n = 15,792) found a significant increase in SVB in the early pushing group (within 30 minutes of confirmed full dilation) in their cohort of nulliparous women, both for those with (OR (95%CI) 1.19 ( 1.04–1.37); OR (95% CI) adjusted 1.20 (1.05–1.38) and without (OR (95%CI) 1.67 (1.03–2.70); OR 95% CI adjusted 1.71 (1.06–2.78)) epidural analgesia⁷⁹.

A Canadian RCT found SVB to be more common in the delayed pushing group (57.5% vs 52.7%, RR 1.09 [95% CI, 1.00-1.18], however the effect size was small and statistical significance likely to have been due to the large sample size (n = 1862)⁸⁰. They also observed a reduction in ‘difficult births’, which included second stage CB and certain types of instrumental birth, across all difficult birth categories, with the exception of CB for fetal distress⁸⁰.

The duration allowed for pushing was explored. A US study found a significantly decreased chance with each subsequent hour to have a SVB for an infant with no signs of asphyxia (interval 1–2 hours: adjusted odds ratio [OR], 0.4; 95% CI 0.3–0.6; interval 2–3 hours: adjusted OR, 0.1; 95% CI, 0.09–0.2; pushing > 3hours: adjusted OR, 0.03; 95% CI, 0.02–0.1; reference: pushing < 1 hours)⁸¹.

FETAL

Three key areas relating to fetal factors were identified in the research: positioning, head circumference and method of fetal heart rate monitoring during labour. To determine if fetal positioning, specifically left occipitoanterior (OA), at the onset of labour is associated with mode of birth, and increased likelihood of SVB⁸², 1250 of 1647 eligible women at a single, publicly funded maternity hospital in the UK had a valid transabdominal scan at labour onset. After adjustment for confounders, there were no statistically significant differences between outcomes for OA, occipitotransverse (OT), or occipitoposterior (OP) positions, demonstrating that at the point of labour onset, fetal occiput presentation is not a predictor for SVB (OR 0.864 (95% CI, 0.617–1.209); P = 0.394).

However, once labour is established fetal occiput position may be influential. In a large US population study, labouring women with a fetus in a persistent OP position had a SVB rate of only 26%⁸³. Narrow suprapubic arch angles have been proposed as a mechanism for this sub-optimal positioning. An Italian cohort study found the narrow angle group were more likely to have persistent OP fetal position at birth (104.4 ± 16.8◦ vs 116.4 ± 11.9◦; P < 0.0001)⁸⁴.

Fetal position at the commencement of second stage is also known to influence mode of birth. Fetal head position at full dilatation was also associated with mode of birth, with OT or OP positions more likely to result in an assisted vaginal birth. A French RCT aimed to trial prophylactic manual rotation at the early second stage, and determine association with operative birth⁸⁵. Women with a singleton term pregnancy, having both OP or OT position and full dilation confirmed by ultrasound then received epidural analgesia and were randomised to a trial of prophylactic manual rotation or usual care. Manual rotation reduced women’s likelihood of an operative birth significantly (29.4% [37 of 126] vs 41.2% [54 of 131];P = .047)⁸⁵, and also reduced the duration of the second stage.

Neonatal head circumference (HC) might contribute to birth outcomes more than fetal birthweight. In an Israeli cohort of 6343 first births of normal birthweight neonates, large head circumference was more strongly associated with a threefold increased likelihood for all forms of operative birth and resultant decrease in SVB than high fetal birth weight (CB: aOR, 3.08; 95% CI, 2.52–3.75 and AVB: aOR, 3.03; 95% CI, 2.46–3.75)⁸⁶. Similarly, an Australian study found that sonographic fetal HC ≥ 90th percentile decreased SVB likelihood, with an increased likelihood of CB OR 2.77 (95% CI: 1.36– 5.62; P = 0.005), compared with those who have a HC < 90^th87. Although not examined discretely fetal head engagement may contribute to mode of birth. In another Australian prospective cohort study of 202 nulliparous women, a composite measure with the co-variates maternal age, BMI, first-degree relative CB, Bishop Score ≤ 2, cervical length and bladder position on Valsalva and fetal head engagement had an 87% accuracy in predicting vaginal birth in their small cohort⁸⁸. A Dutch study similarly found that predictive factors for operative birth were found in physical/medical indications, such as fetal distress (as identified through continuous electronic fetal monitoring) (aOR 49.11 (7.32–329.31), non-occipitoanterior position (3.08 (1.25–7.56)) and higher fetal weight (2.68 (1.40–5.14))²⁷.

Continuous Electronic Fetal Monitoring (CEFM) for low-risk women in spontaneous labour is not recommended⁸⁹, and a Swedish prevalence study of two historical cohorts examined the de-implementation of unnecessary interventions (admission CTG, augmentation with synthetic oxytocin)⁹⁰. Whilst SVB rates were not significantly affected (2009 (85.6%) compared with 2012 (87.4%)), unnecessary augmentation and CEFM were reduced⁹⁰. A large Australian cohort study (n = 1600) found that during a period where Centralised CEFM was unavailable, there was no effect on the prevalence of SVB for nulliparous women who utilised CTG monitoring during labour (CFM group 42.1% V No CFM group 48.4% (p = 0.79))⁹¹.

Declining rates of SVB by nulliparous women, especially in high-resource settings, require urgent attention. This review has methodically searched, appraised and coalesced available literature into key areas where opportunities may lie to address modifiable factors, at maternal, clinical care and fetal domains.

Whilst demographic factors of advancing maternal age and ethnicity are non-modifiable and known to influence mode of birth^17,18,20, much literature examined BMI in the pregnant population and its association with mode of birth⁹². The existing association of obesity and pregnancy comorbidities, such as gestational diabetes, cumulatively effect mode of birth. Studies in our review found a decreased likelihood of SVB for women classed as obese^23,24 and this has a gradient effect with increasing severity of obesity²⁴. Mechanisms to address this prior to conception are ideal⁹³, however once pregnant, women should be offered strategies to mitigate the impact of this factor on their opportunity to birth spontaneously.

Women’s intentions and preferences regarding mode of birth, were addressed constructively in several Middle Eastern settings, where maternal request for CB is high³³. Applying rigorous psychological theories (such as the Theory of Planned Behaviour and the Health Belief Model), in programs to address intention regarding normal birth were effective, and in some studies shown to influence mode of birth. However, these studies were generally small singles-site settings and findings need to be interpreted with caution. An emerging associated area of focus is fear of childbirth⁴³, and screening women in order to address high levels of fear is another potentially modifiable factor that could mitigate maternal request for CB and stall declining SVB trends.

High quality childbirth education, providing instruction antenatally to women on breathing techniques, and yoga and Pilates based programs have been found to improve SVB as an outcome. Similarly, reinforcing the value of regular, moderate exercise was also effective. These interventions are largely low cost, acceptable to childbearing women and could be applied as an adjunct to traditional maternity services.

For medically low-risk women, the upscaling of midwifery continuity of care models has potential for a profound effect on SVB⁴⁸,⁴⁴. The mechanism for this significant improvement could be attributed to the known influence of continual labour support on mode of birth⁵³ and the resultant decrease in frequency of epidural analgesia – an area strongly associated with assisted vaginal birth⁶⁶. Maternal intentions may also be influential here, as many studies included in our review contained a ‘self-selected’ cohort, however RCT evidence supports improvement in SVB for low-risk women. Avoiding admission to birthing services in the early, latent phase of labour⁵⁵, may also be a mechanism by which care providers enhance SVB rates. Similarly, avoiding unnecessary intervention in nulliparous women, especially those labouring spontaneously, may also be influential. Interventions, such as augmentation and epidural analgesia were found to greatly decrease likelihood of an SVB when women were low-risk and in spontaneous labour.

While evidence of the effect of some factors was confirmed (for example, having a known care provider) evidence for other factors was disparate. Induced or expectant onset of labour, for low-risk women, was compared in the ARRIVE trial, and women who were induced birthed spontaneously more often (74 v 69%), however the authors did not report if this difference as statistically significant or not⁵¹. Conversely, large cohort studies have found that induction of labour for low-risk nulliparous women, either made no difference⁹⁴ or significantly increased women’s odds of a caesarean birth⁹⁵. This area requires further research, and in our review, findings do not support a routine approach to offering induction of labour as a mechanism to increase SVB.

Advancing technologies have led to an emerging area of research on maternal pelvic tissue anatomy and its relationship to fetal positioning and mode of birth. The sample sizes within the included studies were small, however these data could be considered when providing woman-centred counselling and decision-making regarding mode of birth, when more complex considerations arise. For example, development of algorithms on fetal biometry, maternal age, ethnicity, pregnancy risk status and women’s intentions regarding mode of birth. However, feasibility across largely publicly funded health services and the obstetric workforce would also need to be considered regarding the advanced technical skills required for such investigations.

Labour dystocia remains the primary reason for intrapartum operative birth⁹⁶ and strategies to optimise labour progression hold promise. During the first stage of labour, long latent phase, maternal fatigue, and sub-optimal fetal positioning were found to reduce likelihood for an SVB, and treatments of sodium bicarbonate when labour stalls were effective in increasing SVB. Similarly, mechanisms to enhance labour progression during second stage (such as manual rotation, supine, or lateral maternal positioning and early rather than delayed pushing) are effective, specifically for women with regional analgesia. Further, the optimisation of non-pharmacological pain relief, such as warm water immersion has been associated with reduction in need for epidural analgesia⁹⁷, again a significant factor associated with reduced likelihood of SVB in nulliparous women.

We acknowledge that the safe reduction of unnecessary CB has been the focus of large-scale reviews and clinical guidance, and evidence cited in this work will overlap with our review. To differentiate, we have reported strategies which have a direct impact on enhancing SVB and application of synthesised evidence into practice provides an alternate path to affect change. The findings of our review are relevant to women, their partners and support people, obstetricians and midwives and point to an informed partnership between women and the interprofessional maternity care team as a means to enhance SVB outcomes. The range of SVB rates (13%-99%) reported across the 78 included studies testify to the complexity of the issue and opportunity for innovation to address this trend.

Strengths and limitations

Given the volume of literature, we deliberately excluded studies assessed as low quality as our intention is that this review will inform approaches to enhance SVB outcomes in maternity care. Many high-quality studies did not differentiate between SVB rates in nulliparous and multiparous women or did not differentiate between assisted and spontaneous vaginal birth and, as such, could not be included in our review. Future research should distinguish parity and birth types as extrapolating this data would contribute to our understanding of specific factors that influence SVB in nulliparous women. Furthermore, approximately 1/3 of studies did not meet the pragmatic 50% threshold set for the quality appraisal. Common areas where study quality was sub-optimal included absence of consumer input, reliability and validity of data collection methods, and ethical concerns regarding consent.

Contemporary maternity care operates within complex systems. Downward nulliparous SVB trends persist. Our review highlights key areas where quality research suggests improvements can be forged, across individual maternal, clinical care and fetal domains. We argue that the imperative to get the first birth right is critical to women’s ongoing physical and psychological wellbeing. These span preconception to imminent birth and arguably applied to individual women’s circumstances can work to ‘turn the tide’ one first birth at a time.

Ethical approval and consent: NA; Consent for publication: NA; Availability of data and material: NA; Authors' contribution: LK conceptualised the review. All authors participated in article screening, critical appraisal, and data extraction. LK, FB, RN and RS drafted the manuscript and all other authors reviewed and endorsed the final draft. Acknowledgement: The authors would like to acknowledge Ms Sue Bethune, health sciences librarian for her invaluable support in conducting the searches; Funding: We would also like to acknowledge the Study, Research and Education Trust Fund, Sunshine Coast Hospital and Health Service and the Wishlist Health Foundation for their financial support of this study. Conflict of interest: Nil to declare. Author's information: see title page.

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Factors associated with spontaneous vaginal birth in nulliparous women at term: a systematic review

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Abstract

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Introduction

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Study Selection

Results

MATERNAL

Maternal characteristics

Maternal mindset

Maternal preparation

CLINICAL CARE

Models of care

First stage of labour

Second stage of labour

FETAL

Discussion

Strengths and limitations

Conclusion

Declarations

References

Appendix

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