Declining birth weight and changing practices

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Changes in obstetric practice over the past 20 years may have led to declining birth weights, according to new research from Demography. Mean birth weight in the United States has declined since the 1990s, and while it stabilized in the 2010s, it has not increased. 

Methods
The study used restricted National Vital Statistics System data which linked birth/infant death data for 1990 to 2013. The authors analyzed trends in obstetric practices, gestational age distributions, and birth weights among first-birth singletons born to US non-Hispanic white, non-Hispanic black, and Latina women. The data sample included 1,001,976 births in 1990, 1,130,812 births in 2013 and 23,027,689 births in all years between 1990 and 2013. The authors also coded obstetric intervention status into four groups: induced labor with cesarean delivery, induced labor with vaginal delivery, labor not induced with cesarean delivery, and labor not induced with vaginal delivery. To predict yearly average birth weight among US first-birth singletons, an OLS regression model was applied. 

Findings
The authors found that the average birth weight for US first-birth singletons in this sample was 3,314.5 g in 1990. By 2013, it had declined by more than 67 g to 3,247.2. Also observed were differences in maternal, behavioral, and birth characteristics between 1990 and 2013. Maternal age has increased considerably in the United States; 30 to 34-year-old mothers represented 13.1% of births in 1990 and that number jumped to 18.9% by 2013. An increasing proportion of first-birth singletons have been to higher-educated, unmarried, and Latina women. 

Compared to mothers in 1990, mothers in 2013 were more likely to have gained excessively low or excessively high gestational weight, were less likely to have smoked during pregnancy, and were more likely to have had diabetes or hypertension. Compared to births in 1990, labor onset in 2013 was much more likely to have been induced and delivery was more likely to have been cesarean. Gestational ages of first-birth singletons in 2013 were much less likely to have been more than 40 weeks and much more likely to have been 37 to 39 weeks than in 1990. 

Through their OLS model, the authors identified three notable patterns in the gestational age distributions of 1990 and 2013. First there were no substantive differences between the 1990 and 2013 rates of preterm birth (PTB), likely attributed to the sample size including first-birth singleton pregnancies which are generally not high-risk pregnancies.  Second, compared with 1990, a much larger proportion of births in 2013 were during gestational weeks 37 to 39. In 1990 and 2013, about 37.9% and 48.5% of births, respectively were during those gestational weeks. Lastly, the proportionate increase in first-birth singletons between gestational weeks 37 and 39 was offset by sizable decreases in births beyond gestational week 40. In 1990, nearly 29% were born beyond gestational week 40 but only 18% were born at these older gestations in 2013. 

In regard to obstetric practices, the authors found that the likelihood of a non-induced vaginal delivery declined slightly among PTBs and dramatically declined among births at gestational ages ≥ 39 weeks. The modal gestational age of non-induced vaginal births shifted down from week 40 in 1990 to week 39 in 2013. For non-induced cesarean delivery, the authors observed very little substantive difference between 1990 and 2013. However, there was a shift in gestational age distribution. Compared to 1990, the likelihood for a non-induced cesarean delivery in 2013 was higher for all gestation ages < 40 weeks and substantially lower for all gestational ages ≥ 40 weeks. 

Conclusions
Taken together, the authors believe that these findings suggest that obstetric interventions were the underlying cause for the changes in the gestational age distributions. Furthermore, they suggest that the recent trends in US birth weight may be correlated to the shifts in gestational age resulting from changes in obstetric interventions, and that if the rates of obstetric practices had not changed during this period, then the average US birth weight would have increased over time.

The authors note several limitations to this study, including no way to measure changes in obstetric decision-making processes, an inability to measure the possible reasons for the increased likely interventions, and limited information on maternal behaviors, characteristics, and risk factors for obstetric interventions. However, they believe that despite these limitations, their findings indicate that the puzzling declines in US birth weight may actually have a simple answer. Future research should explore the institutional and social mechanisms that lead to the high rates of labor induction and cesarean deliveries among US births.