Our retrospective analysis of births planned in the AMU at our institution, a level three university hospital, confirms the non-inferiority of this model compared to standard obstetric care for selected maternal and newborn outcomes. Additionally, the outcome of women transferred during or immediately after labor serves as an indirect confirmation of the medical safety of this model of care and the appropriateness of the transfer checklist. Our findings apply to healthy women entering labor after uneventful pregnancy. To the best of our knowledge, this is the first study reporting on obstetric and perinatal outcomes of women intending to give birth in an AMU in Germany.
(a) Comparison between the study and control group
The comparison between the study and control group revealed a non-significant trend towards higher spontaneous and lower instrumental vaginal births. In both groups, the CS rate was below 10%, illustrating the low risk of our cohort.
We analyzed epidural anesthesia and episiotomy rates. Both interventions were less commonly performed in the study group. This finding is in accordance with the existing literature (3)(4).
Causes for the difference in the rate of higher degree perineal lacerations may include obstetric and newborn factors: More women in the study group were overweight or obese, and more than one third of the injuries (38.5%) that occurred after transfer to standard obstetric care were associated with instrumental vaginal births. Additionally, newborns in the study group had a higher mean birthweight, and newborns with birthweight ≥ 4,500 g belonged almost exclusively to the study group (91.7%). Instrumental births and macrosomia are established risk factors for higher-order obstetric lacerations.
Severe neonatal morbidity or even mortality are rare events in high-income countries. For example, severe metabolic acidosis, defined as umbilical cord arterial pH < 7.00 in singleton term newborns occurred with an incidence of 0.21% in Germany in 2017 (9). The size of our study group was therefore insufficient for a valid comparison of the perinatal outcome in the two models of care. Using a composite outcome, we showed non-inferiority of AMU to standard obstetric care in order to compensate for this shortcoming.
(b) Causes for and outcomes of transferred births
With 50.3% of women being transferred to standard obstetric care during or immediately after birth, our transfer rate was high. Explanations include the high rate of nulliparous women, and the strict transfer criteria. Nulliparity, higher maternal age and birthweight increased the chance of transfer to standard obstetric care. Higher transfer rates of nulliparous women have also been described by the authors of the Birthplace in England Study (36-45% in nulliparous vs. 9-13% in parous women) (6). The effect of parity may also explain the low transfer rate (7.0%) of a recently published Austrian study. Here, only 27% of women were nulliparous (10).
The composition of the three groups (study group, transferred; study group, not transferred; and control group) does not allow for a quantitative comparison of outcomes. We therefore limited our analysis to a descriptive presentation. Overall, primary and secondary maternal and newborn outcomes were comparable between the transferred and the control group. We take this result as indirect proof for the safety of births in our AMU. Additionally, the comparability of maternal and newborn outcomes in the transferred group and the control group may serve as evidence for the adequacy of the transfer checklist.
Various factors limit a direct comparison of our results with other studies. These include differences in study design and methods, e.g. with respect to randomization, analysis according to intended versus actual place of birth, and risk assessment. Furthermore, variations exist in birth settings, since organization of maternal healthcare provision is country-specific; this pertains to the key provider of care during birth (midwife versus general practitioner versus specialist); the location of alongside or freestanding midwifery units with respect to the obstetric unit; and the transfer modalities to obstetric units in case of abnormalities occurring during or after labor.
In the systematic reviews by Bohren et al. 2017 (3) and Sandall et al. 2016 (4) only randomized controlled or cluster-randomized trials were included. One-to-one intrapartum support was compared with ‘usual’ care in any setting for its effect on various obstetric outcomes in the former, midwife-led continuity of care models versus other models of care in the latter analysis. Among other outcome variables, higher rates of spontaneous births, lower rates of regional analgesia, a shorter duration of labor, higher 5-minute Apgar scores, and no difference in the perineal trauma rates were reported in these reviews (3)(4).
Scarf et al. 2018 (11) in their meta-analysis compared maternal and perinatal outcomes by planned place of birth. 28 publications with different study designs and methods were included, illustrating the above mentioned limitations. The reported outcomes were not stratified according to core obstetric criteria including parity, epidural analgesia, and oxytocin augmentation. The transfer rates and outcomes after transfer were not mentioned, nor the profession of the providers looking after women who gave birth in hospitals (11).
Of all non-randomized studies, the ‘Birthplace in England national prospective cohort study’ represents the largest study of its kind with the most rigorous design (6). Maternal and perinatal outcomes of 64,538 low-risk women were prospectively analyzed according to the planned place of birth. With the exception of planned home birth in nulliparous women, the perinatal outcome was comparable in midwifery-led models of care compared to births in an obstetric unit, with less maternal interventions in the former group (6).
More recent European studies share our retrospective design, however, study groups or control groups are different. Gidaszewksi et al. 2019 investigated nulliparous women only and compared caseload with standard midwifery-led care (12). Jepsen et al. 2018 included at-risk women and compared caseload with standard midwifery care (13).The study design applied by Bartuseviciene et al. 2018 does not allow the calculation of the number of women who actually received standard obstetric care (14). In the above cited study by Bodner-Adler et al. 2017, the control group was chosen for matching after (successful vaginal) birth, thereby precluding a direct comparison with the study group. Additionally, care in AMU included oxytocin augmentation and i.v.-opioid analgesia (10).
Strengths of our study include the strict inclusion and exclusion criteria applied for both, study and control group; the pre-specified and documented transfer criteria; and the description of the maternal and newborn outcome after transfer from AMU to standard obstetric care.
Limitations of our study pertain to its retrospective design and the size of the study and control group, which precludes the analysis of rare maternal or newborn complications. Additionally, the high transfer rate has to be borne in mind for comparison with other studies.