CU Population Center

Institute of Behavioral Science

Worth the Weight? Recent trends in obstetric practices, gestational age, and birth weight in the U.S.

CUPC affiliates, Andrea Tilstra (pre-doc, Sociology) and Ryan Masters (faculty, Sociology), recently published their research Worth the Weight? Recent Trends in Obstetric Practices, Gestational Age, and Birth Weight in the United States, in Demography.

Between 1990 and 2013, average weight at birth in the United States has dropped from 7.31 pounds to 7.16 pounds, an average difference of 0.15 pounds (2.4 ounces). This trend has alarmed researchers given previous findings indicating low birth weight is associated with a variety of adverse birth outcomes and poor early life health. In Worth the Weight, Tilstra and Masters suggest these trends are likely due to changes in gestational age resulting from changes in obstetric practices since the early 1990s.

To come to this conclusion, they use restricted National Vital Statistics System data linked birth/infant death data for 1990–2013 to show that the likelihood of births occurring after the 40th week of pregnancy dropped significantly between 1990 and 2013, while the likelihood of early term births (37-39 weeks) drastically increased. At the same time, the likelihood of induced labors and cesarean deliveries increased across all gestational ages. Specifically, nearly 18% of births from non-induced labors and vaginal deliveries at later gestational ages were replaced with births occurring at earlier gestational ages from obstetric interventions.

To test their hypothesis that changes in gestational ages resulted from changes in gestational practices, the researchers use simulation techniques to estimate changes in birth weight distributions if obstetric practices had not changed between 1990 and 2013. Results from these simulations show that rather than decreasing over time, U.S. birth weight would have increased over time had obstetric practices not changed. These findings, they say, “strongly indicate that recent declines in U.S. birth weight were due to increases in induced labor and cesarean delivery at select gestational ages.”

Tilstra’s and Masters’ research sheds light on how increases in obstetric interventions in the United States have potentially wide-ranging unintentional consequences for women’s and infants’ health. They hope that future research considers how norms, preferences, and practices at various levels of U.S. social life work to influence, and even encourage, obstetric interventions.