Trends in childhood viable pregnancy and risk of stillbirth in the United States

There is limited data on the trends of childhood viable pregnancy and the risk of stillbirth in the United States. Our study assessed the trends in childhood viable pregnancy and associated stillbirth rates over the previous three decades, as well as the risk of stillbirth in these highly vulnerable child mothers aged 10–14 compared with teen mothers aged 15–19. We conducted a population-based retrospective cohort study that used birth datasets, fetal death datasets, and the US population census data: 1982–2017. To assess the association between various sociodemographic and maternal comorbidities and stillbirth, we generated adjusted hazard ratios (AHR) from Cox proportional hazards regression models. From 1982 to 2017, viable pregnancy rates declined among children (from 0.3/1000 to 0.06/1000 population) and teens (from 40.5/1000 in 1982 to 18.1/1000). Overall, there were declines in the stillbirth rates in both teens (15–19 years old) and child mothers aged 10–14 years, but the rate remained consistently higher among child mothers vs. teen mothers (14 per 1000 vs. 8 per 1000 viable pregnancies). Compared to teen mothers, childhood pregnancy was modestly associated with an elevated risk for stillbirth (AHR = 1.09; 95% CI = 1.05–1.12). Other factors significantly associated with increased risk of stillbirth included maternal race, preterm birth, arterial hypertension, diabetes, and eclampsia (P<0.0001). Conclusion: Childhood pregnancy may be a risk factor for stillbirth. This is the first study to assess the trends in childhood viable pregnancy and the associated stillbirth rates in the United States. These findings further underscore the need for sustained efforts and policies to prevent pregnancies in the early years of reproductive development. What is Known: • Childhood pregnancy, defined as pregnancy among 10–14 year-old females, may be associated with a number of pregnancy complications and adverse pregnancy outcomes, including preterm delivery, low birth weight, and infant mortality. • Structural disparities in socioeconomic status and access to healthcare place some teenagers at high risk of teen pregnancy. What is New: • Our study shows the trends in childhood pregnancy over the previous three decades; overall, there were declines in the stillbirth rates in both child mothers aged 10–14 years and teen (15–19 years old) mothers, but the rate remained consistently higher among child mothers. • Child mothers aged 10–14 were more likely to experience stillbirth than teenagers, and Black mothers had an increased risk of stillbirth than White mothers—all of which underscores the effects of structural health disparities. What is Known: • Childhood pregnancy, defined as pregnancy among 10–14 year-old females, may be associated with a number of pregnancy complications and adverse pregnancy outcomes, including preterm delivery, low birth weight, and infant mortality. • Structural disparities in socioeconomic status and access to healthcare place some teenagers at high risk of teen pregnancy. What is New: • Our study shows the trends in childhood pregnancy over the previous three decades; overall, there were declines in the stillbirth rates in both child mothers aged 10–14 years and teen (15–19 years old) mothers, but the rate remained consistently higher among child mothers. • Child mothers aged 10–14 were more likely to experience stillbirth than teenagers, and Black mothers had an increased risk of stillbirth than White mothers—all of which underscores the effects of structural health disparities.


Introduction
Although the teen pregnancy rate has decreased over the past two decades in the US, it still remains an important public health issue [1]. The teen pregnancy rate in the US is higher than in other industrialized Western countries [2], and disparities exist by race/ethnicity as well as geographic region. [3] In 2017, the overall rate of teen pregnancy among women aged 15-19 years old was 18.8 per 1000 women, with Black and Hispanic teens having over twice the pregnancy rate of their White counterparts. [4] Structural disparities in socioeconomic status and health place some teenagers at high risk of teen pregnancy. [5] As a consequence, pregnancy during the teenage years not only causes problems for the adolescent mother but also creates issues for the child and society at large. The children of teen mothers have an increased risk of teen pregnancy, higher high school dropout rates, greater chances of incarceration during their teenage years, and higher unemployment as young adults. [6] Childhood pregnancy was previously defined as pregnancy among 10-14-year-old females [1,7] and could be associated with a number of pregnancy complications and adverse pregnancy outcomes, including preterm delivery, low birth weight [8][9][10], and infant mortality. [11,12] At the proximal level, the increased risk of neonatal and postneonatal mortality among young teens has been attributed to biological immaturity. [13] While at the distal level, adverse pregnancy outcomes associated with low maternal age are attributed to health predictors, including poor socioeconomic status [14] and racial disparity. [15] Low socioeconomic conditions may impact dietary habits, access to prenatal care, and cigarette smoking or other tobacco use. [14] All of these social determinants of health may increase the risk of adverse birth outcomes among child mothers. Only focusing on individuals' behaviors overlooks the reality that the risk environment shapes individuals' behavior [5]. The distal determinants of health precede the proximal determinants of health; therefore, structural changes at the society level are more effective in reducing inequities and improving the lives of teens and children [5] Since 1981, numerous teen pregnancy prevention policies and programs have been developed and implemented [16]. An example of the national programs includes the Adolescent Family Life (AFL) program that was established in 1981 to provide education and social service to pregnant adolescents; however, the program primarily focused on abstinence-only education [16]. In 2010, the Consolidated Appropriations Act established a new Teen Pregnancy Prevention (TPP) program that funds public and private entities in implementing medically accurate and age-appropriate programs. Furthermore, the Patient Protection and Affordable Care Act provided grants for states to start a Personal Responsibility Education Program (PREP), which provides education on both abstinence and contraception [16]. These policies and program implementation changes in the past decades might have led to an expected change over time, which has not been previously studied. Although the association between childhood pregnancy and the risk of stillbirth is known [1], updated data on the trends of childhood pregnancy in the US is lacking. Accordingly, in this study, we examined the trends in childhood pregnancy and associated stillbirth rates over the previous three decades, as well as the risk of stillbirth in these highly vulnerable mothers compared with teen mothers aged 15-19. The finding of this study has an important public health implication in understanding the efficacy of current policy and programs. It is also essential in shaping future interventions that may need to be tailored toward a certain maternal age or ethnic group that is at high risk of adverse health outcomes.

Methods
We conducted a population-based retrospective cohort study that used birth datasets and fetal death datasets made available by the Centers for Disease Control and Prevention, the National Vital Statistics System, the National Center for Health Statistics, and the US population census data from the United Nations. [17] The birth dataset had data on all births occurring within the US, while the fetal death dataset What is Known: • Childhood pregnancy, defined as pregnancy among 10-14 year-old females, may be associated with a number of pregnancy complications and adverse pregnancy outcomes, including preterm delivery, low birth weight, and infant mortality. • Childhood pregnancy may be linked to stillbirth. • Structural disparities in socioeconomic status and access to healthcare place some teenagers at high risk of teen pregnancy. What is New: • Our study shows the trends in childhood pregnancy over the previous three decades.
• Overall, there were declines in the stillbirth rates in both child mothers aged 10-14 years and teen (15-19 years old) mothers, but the rate remained consistently higher among child mothers. • Child mothers aged 10-14 were more likely to experience stillbirth than teenagers, and Black mothers had an increased risk of stillbirth than White mothers-all of which underscores the effects of structural health disparities.
contained information on fetal death. The information available in these datasets was extracted from the birth and death certificates, respectively. The datasets included sociodemographics, health characteristics, and maternal risk factors associated with each live birth and stillbirth. For our study, we restricted our analyses to singleton births within the gestational age from 20 to 42 weeks for the years 1982 through 2018 from the birth dataset and 1982-2017 from the fetal death dataset, occurring to mothers aged 10-19 years. We excluded all live and stillbirths outside of the gestational age and maternal age range stated above.

Study variables
We included the variables that were commonly available in the datasets for most of the study period. Our exposed group, childhood pregnancy, was defined as a pregnancy that resulted in singleton live birth or stillbirth among mothers 10-14 years of age. For our comparison group, we used mothers ages 15 to 19, who are also referred to as adolescent mothers. [18] Our primary outcome, stillbirth, was defined as the intrauterine death of a fetus at ≥20 weeks of gestation. [18] Covariates included in the study were (1) race, which was categorized as Whites, Blacks, and Others (which included all races except Whites and Blacks). We used this categorization as it was available for all years over the study period; (2) birthplace, classified as hospital, home, and others; (3) delivery method, categorized as vaginal and C-section; (4) birth attendant, classified as certified medical professionals including medical doctors, nurses, midwives, and others; (5) gestational age (which was measured based on the date of last menstrual period (LMP)), categorized as preterm (i.e., <37 weeks or term, i.e., ≥37 weeks); (6) maternal diabetes-the variable for diabetes was given without segregation between prepregnancy and gestational in the birth dataset until the year 2003 and in the fetal death dataset until 2013. So, we combined the variables to encapsulate any type of maternal diabetes; (7) chronic arterial hypertension-the variable for chronic arterial hypertension was changed to "pre-pregnancy arterial hypertension" as from 2004 in the birth dataset and as from 2014 in the fetal death dataset. Therefore, we combined the two variables and designated it as "chronic arterial hypertension" for this study; (8) pregnancy arterial hypertension-the variable for pregnancy arterial hypertension was changed to "gestational arterial hypertension" as from 2004 in the birth dataset and as from 2014 in the fetal death dataset. We decided to combine the two variables and called it "pregnancy arterial hypertension" for this study; (9) eclampsia-the variable for eclampsia was changed to "arterial hypertension eclampsia" as from 2004 in the birth dataset and as from 2014 in the fetal death dataset. Therefore, we combined the two variables and called it "eclampsia" for this study.

Statistical analysis
We calculated the rate of childhood viable pregnancy for each year by dividing the number of viable pregnancies (live birth and stillbirth) among girls aged 10-14 years by the total number of females aged 10-14 years in the US population. We compared these with similarly derived rates for adolescent mothers ages 15-19 years. This information was extracted from 1982 through 2017 from the birth and fetal death datasets (numerator) as well as the US population census (denominator). Similarly, to derive the rate of childhood live birth for each year, we divided the number of live birth among girls aged 10-14 years by the total number of females aged 10-14 years in the US general population. Equivalent computation was performed for the comparison group (15-19 years old mothers). Live birth rates from 1982 to 2018 were computed to allow for comparison with previous studies that defined pregnancy rates using only live births. We also calculated stillbirth rates for both age groups by dividing stillbirth by the sum of live birth and stillbirth and then multiplying by 1000. The analyses involving stillbirth were restricted to the years 1982-2017 as the fetal death data were available only until 2017. We used joinpoint regression, which is a modeling technique, to detect the change in the rate of events over time. [19] We also calculated the average annual percentage change (AAPC) for trends in rates of viable pregnancies, live birth, and stillbirth in both child and teen mothers over the study period. W e c o n d u c t e d b i v a r i a t e a n a l y s e s t o u n c o v e r sociodemographic and maternal risk factors among child and adolescent mothers. We also examined the factors potentially contributing to live birth or stillbirth among children using chisquare test that detects differences in proportion. To determine independent associations across the various sociodemographic and maternal comorbidities, including diabetes, chronic arterial hypertension, pregnancy arterial hypertension, and eclampsia versus stillbirth, we generated hazard ratios (HR) and adjusted hazard ratios (AHR) from the survey Cox proportional hazards regression models. In the adjusted models, we controlled for race, place of birth, delivery method, attendant at birth, gestational age, maternal diabetes, chronic arterial hypertension, pregnancy arterial hypertension, and eclampsia, after removing missing values from all the covariates (analytic sample=6,727,808). All tests of hypothesis were two-tailed with a type 1 error rate set at 5%. Because the study was performed using publicly available de-identified data, it was approved as exempt by the Institutional Review Board of Baylor College of Medicine.

Results
We analyzed pregnancy data collected from 14,224,602 mothers over the 36-year study period. Of these mothers, 260,928 (1.8%) were child mothers aged 10-14, and 13,963,674 (98.2%) were teens aged 15-19 years old. Table 1 describes the sociodemographic characteristics of mothers in this study stratified by age, 10-14 years and 15-19 years. Overall, most of the mothers were White (57.4%), had a term delivery (85.3%), and delivered their baby in the hospital (90.9%) through vaginal birth (55.0%). Child mothers were less likely to be White than teen mothers (40.8% versus 57.7%). Furthermore, mothers aged 10-14 were more likely to have a preterm birth than teen mothers (24.4% versus 14.5%). The prevalences of chronic arterial hypertension (0.3%), pregnancy arterial hypertension (3.1%), diabetes (1.8%), and eclampsia (0.3%) were very low among study participants though approximately half of the mothers (49.9%) had missing data on these select medical conditions. Figure 1 presents temporal trends in the incidence of viable pregnancy among children and teens. From 1982 to 2017, the rate of viable pregnancy among children declined from 0.3/1000 to 0.06/1000 population, representing a drop of 80.0%. Among  Figure 1) were very similar to those for viable pregnancies described above. Table 2 shows the sociodemographic characteristics of child and teen mothers stratified by live birth and stillbirth. Out of the total number of births in both groups (N = 14,224,602), 0.8% (N=110,134) were stillbirth. About 3.0% (N= 3552) of the total stillbirth occurred among child mothers The incidence of stillbirth in child and teen mothers was 14 per 1000 and 8 per 1000 viable pregnancies, respectively. Among children, the incidence of stillbirth was significantly higher among mothers of Black race (1.7%) and those who had a cesarean section (3.1%). For teens, the incidence of stillbirth was higher among mothers of other races (1.3%) and those who had a vaginal birth (1.0%). Among child and teen mothers with missing data for birth attendants, the incidence of stillbirth was 89.8% and 89.4%, respectively. Regardless of age group, the stillbirth rate was very high among mothers who had preterm birth and chronic arterial hypertension. The risk of stillbirth among preterm fetuses of children and teen mothers was over 20 times more frequent compared with term fetuses. Figure 2 shows the trends in stillbirth rates among childhood and teenage mothers. Overall, there was a decline in stillbirth rate in both groups from 1982 to 2017, but the rate remained consistently higher among child mothers. The stillbirth rate in mothers aged 10-14 dropped from about 14.8 to 11.0 per 1000 pregnancies, representing a 25.7% decline. Among teenage mothers, there was a 34.0% decrease from 10.0 to 6.6/1000 pregnancies.
In Table 3, we present the results of the unadjusted and adjusted Cox proportional hazards model for the association .between demographic and health characteristics versus the risk of stillbirth. In the unadjusted model, mothers aged 10-14 were almost twice as likely to experience stillbirth (HR = 1.86; 95% CI =1.81-1.93). However, this risk was reduced considerably in the adjusted model (AHR = 1.09; 95% CI = 1.05-1.12). Black mothers had a 19% increased risk of stillbirth than White mothers (AHR = 1.19; 95% CI = 1.18-1.21). In addition, the risk of stillbirth was more than 4 times as high among mothers with preterm birth compared to those with term delivery (AHR = 4.25; 95% CI = 2.45-7.46). Furthermore, chronic arterial hypertension was a significant risk factor for stillbirth. Mothers with chronic arterial hypertension had a risk of stillbirth that was more than 5 times as high compared to those without chronic arterial hypertension (AHR=5.82; 95% CI= 5.58-6.07). Mothers who delivered through cesarean section had a 74% lower risk of stillbirth compared to those who had a vaginal delivery (AHR=0.26; 95% CI= 0.25-0.28). Other factors significantly associated with increased risk of stillbirth included diabetes and eclampsia (P<0.0001).

Discussion
In this study, we found childhood pregnancy to be associated with greater risks for stillbirth compared to older teenagers (15-19 years). Though this finding was statistically significant, the unadjusted risk reduced by about 40% due to adjustment for potential maternal health and healthcare-related confounders. Preterm birth mainly accounted for this difference because its incidence was considerably higher in the child than teen mothers (24.4% vs. 14.5%). Additionally, our results suggest that young mothers with chronic arterial hypertension or preterm birth were more likely to experience stillbirth. Although the rate of childhood pregnancy had declined over time, it had remained stable since 2016 and was an important risk factor for stillbirth. We also observed a steady decline in stillbirth rates in both childhood and teenage groups, but the rate remained consistently higher among child mothers. Other predictors of stillbirth in our study included vaginal delivery, birth attendants, diabetes, eclampsia, and the Black race.
Our results corroborate the findings of previous studies that showed similar trends in childhood and teen pregnancy and adverse pregnancy outcomes. [4,[20][21][22][23] In contrast to most of the earlier studies in which pregnancy was defined based on live births, we also conceptualized pregnancy as the total number of live birth and stillbirths. However, it is logical that the rates in live birth will be a close approximation of the viable pregnancy rates because stillbirth is a rare occurrence. In the current study, we also observed a modestly increased risk of stillbirth among child mothers compared to teenage mothers. The increased risk of stillbirth with childhood pregnancy may be explained by different physiologic processes that result in adverse birth outcomes. One of these is the biological immaturity of the young child who is still growing, which triggers fetal-maternal competition for nutrients, thereby compromising fetal growth, development, and survival as the pregnancy progresses. [23] Another factor is the social environment, which suggests that poor pregnancy and birth outcomes among teenagers are resultant effects of their disadvantaged social environment. [13,23] We also observed a steady decline in pregnancy and stillbirth rates in childhood and teenage groups, similar to recent reports. [4,20] This could be due to the 1981 and 2010 initiatives spearheaded by the Office of Population Affairs (OPA) at the Department of Health and Human Services (DHHS) that address social determinants of teen pregnancy, such as the use of different birth control methods and abstinence from sexual activities. [4] The time in the decline of teen pregnancy observed in our dataset match the timing of when those initiatives were first introduced. This means that the efforts have led to increased access to prenatal care and have prompted attention to medical conditions during pregnancies. [4,21] Another factor is that adequate knowledge on the timing of stillbirth-specific risk factors may help in reducing antepartum and intrapartum stillbirth risks through close monitoring and prompt intervention. [4] Adolescents with adequate sexual and reproductive health literacy have been shown to be less likely to experience single and recurrent teenage pregnancies. [24,25] One of the strengths of the study is that it is a US population-wide database, and the results will be minimally affected by selection biases. Despite this, the study has limitations, including our inability to assess the contribution of certain behavioral characteristics such as illicit drug use, high-risk sexual behaviors, and sexually transmitted infection (STI) status. Due to the unavailability of data, we also could not evaluate the effect of some structural determinants of health, such as the experience of discrimination, wealth, and access to healthcare. [14,15] In addition, the reliability and validity of birth certificate information differ from element to element; information on birth weight, delivery method, and insurance are more reliable than information on labor, maternal risk, or pregnancy complications. Also, since our study spanned multiple decades, we observed that in the initial years, maternal comorbidities were not consistently captured, resulting in some missingness. We were unsure whether the missing comorbidity meant failure to capture the information or the absence of the disease. Therefore, we excluded the records with missing information on the comorbidities in our regression analysis, Our findings suggest that childhood pregnancy (10-14 year olds) may be associated with a higher likelihood of stillbirth when compared with teenage pregnancies (15)(16)(17)(18)(19) year olds). We also found that pregnancy and stillbirth rates have declined among teen and child mothers over the previous three decades. Our findings from the regression model, in which most of the higher unadjusted risk was ameliorated when maternal health and healthcare-related factors were taken into account, suggest the need for structural changes in the health system to ensure mothers <14 receive bestpractice care during pregnancy and the birth. In addition to having access to adequate and quality healthcare, young individuals should also have access to low-cost and comprehensive reproductive health education and contraception so that they can make informed decisions about their health and their future children [26]. Therefore, these findings underscore the need for sustained efforts and policies to reduce structural barriers, improve healthcare access to vulnerable populations, and prevent pregnancies in the early years of reproductive development Availability of data and materials Data will be made available upon request Code availability Codes will be made available upon request Authors' contributions Sahra Ibrahimi conceived the project idea and drafted the manuscript. Deepa Dongarwar assisted with drafting the manuscript and performing the data cleaning and data analyses.
Dr. Korede Yusuf assisted with drafting the manuscript and providing feedback.
Dr. Sitratullah Maiyegun assisted with drafting the manuscript and providing feedback.
Dr. Hamisu Salihu provided feedback and supervised the study.

Declarations
Ethics approval N/A

Conflict of interest
The authors declare no competing interests.