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Prepregnancy Obesity and Birth Outcomes


Objective To investigate the association between prepregnancy obesity and birth outcomes using fixed effect models comparing siblings from the same mother. Methods A total of 7496 births to 3990 mothers from the National Longitudinal Survey of Youth 1979 survey are examined. Outcomes include macrosomia, gestational length, incidence of low birthweight, preterm birth, large and small for gestational age (LGA, SGA), c-section, infant doctor visits, mother’s and infant’s days in hospital post-partum, whether the mother breastfed, and duration of breastfeeding. Association of outcomes with maternal pre-pregnancy obesity was examined using Ordinary Least Squares (OLS) regression to compare across mothers and fixed effects to compare within families. Results In fixed effect models we find no statistically significant association between most outcomes and prepregnancy obesity with the exception of LGA, SGA, low birth weight, and preterm birth. We find that prepregnancy obesity is associated with a with lower risk of low birthweight, SGA, and preterm birth but controlling for prepregnancy obesity, increases in GWG lead to increased risk of LGA. Conclusions Contrary to previous studies, which have found that maternal obesity increases the risk of c-section, macrosomia, and LGA, while decreasing the probability of breastfeeding, our sibling comparison models reveal no such association. In fact, our results suggest a protective effect of obesity in that women who are obese prepregnancy have longer gestation lengths, and are less likely to give birth to a preterm or low birthweight infant.

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  1. As is common in survey data, the total net family income variable in the NLSY is self-reported and a significant number of women do not report their income. Rather than exclude them from our analysis, we use self-reports of income to divide women into four categories: high, middle, low, and missing income. Income is adjusted for inflation.

  2. Although the NLSY does not ask directly about gestational diabetes, respondents are asked retrospectively if they have ever been diagnosed with diabetes. We match that with information on her child's year of birth and identified 46 women who were diagnosed with diabetes when they would have been pregnant. If we omit these women from our analysis, as Ludwig and Currie7 do, our results are virtually identical.

  3. Although the rate of c-section observed in the sample is lower than the current national rate of approximately 33 percent, recall that our data are observed from 1979 to 2004. Our annual c-section rates do increase over time similar to those in the U.S. over this period.


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Correspondence to Susan L. Averett.

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Averett, S.L., Fletcher, E.K. Prepregnancy Obesity and Birth Outcomes. Matern Child Health J 20, 655–664 (2016).

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  • Infant health
  • Birth outcomes
  • BMI
  • Maternal obesity
  • Gestational weight gain