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Worth the Weight? Recent Trends in Obstetric Practices, Gestational Age, and Birth Weight in the United States

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Birth weight in the United States declined substantially during the 1990s and 2000s. We suggest that the declines were likely due to shifts in gestational age resulting from changes in obstetric practices. Using restricted National Vital Statistics System data linked birth/infant death data for 1990–2013, we analyze trends in obstetric practices, gestational age distributions, and birth weights among first-birth singletons born to U.S. non-Hispanic White, non-Hispanic Black, and Latina women. We use life table techniques to analyze the joint probabilities of gestational age-specific birth and gestational age-specific obstetric intervention (i.e., induced cesarean delivery, induced vaginal delivery, not-induced cesarean delivery, and not-induced vaginal delivery) to fully document trends in obstetric practices by gestational age. We use simulation techniques to estimate counterfactual changes in birth weight distributions if obstetric practices did not change between 1990 and 2013. Results show that between 1990 and 2013, the likelihood of induced labors and cesarean deliveries increased at all gestational ages, and the gestational age distribution of U.S. births significantly shifted. Births became much less likely to occur beyond gestational week 40 and much more likely to occur during weeks 37–39. Overall, nearly 18% of births from not-induced labor and vaginal delivery at later gestational ages were replaced with births occurring at earlier gestational ages from obstetric interventions. Results suggest that if rates of obstetric practices had not changed between 1990 and 2013, then the average U.S. birth weight would have increased over this time. Findings strongly indicate that recent declines in U.S. birth weight were due to increases in induced labor and cesarean delivery at select gestational ages.

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  1. Gestational-specific life tables for births in 1998 and 2005 as well as for births to non-Hispanic Black, non-Hispanic White, and Latina women are available in the online appendix.

  2. Results from only the first and fourth model are presented here. All other results are available in the online appendix.

  3. Analytic scripts are available in the online appendix.


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We thank the Eunice Kennedy Shriver National Institute of Child Health and Human Development (NICHD)–funded University of Colorado Population Center (Award Number P2C HD066613) for development, administrative, and computing support; and the National Association for Public Health Statistics and Information Systems and the National Vital Statistics Systems for providing data access. We also thank the anonymous referees for providing helpful comments and suggestions, and to jimi adams for his invaluable advice. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NICHD, the National Institutes of Health, NAPHSIS, or the NVSS. Previous versions of this manuscript were presented at the University of Colorado Boulder Sociology Department’s Population and Health Working Group and at the 2018 annual meeting of the Population Association of America in Denver, CO.

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Correspondence to Andrea M. Tilstra.

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Tilstra, A.M., Masters, R.K. Worth the Weight? Recent Trends in Obstetric Practices, Gestational Age, and Birth Weight in the United States. Demography 57, 99–121 (2020).

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