Background Excessive weight gain during pregnancy seems to increase birthweight and the offspring's risk of obesity later in life. However, this association might be confounded by genetic and other shared effects. We aimed to examine the association between maternal weight gain and birthweight using state-based birth registry data that allowed us to compare several pregnancies in the same mother. Methods In this population-based cohort study, we used vital statistics natality records to examine all known births in Michigan and New Jersey, USA, between Jan 1, 1989, and Dec 31, 2003. From an initial sample of women with more than one singleton birth in the database, we made the following exclusions: gestation less than 37 weeks or 41 weeks or more; maternal diabetes; birthweight less than 500 g or more than 7000 g; and missing data for pregnancy weight gain. We examined how differences in weight gain that occurred during two or more pregnancies for each woman predicted the birthweight of her offspring, using a within-subject design to reduce confounding to a minimum. Findings Our analysis included 513 501 women and their 1 164 750 offspring. We noted a consistent association between pregnancy weight gain and birthweight (β 7·35, 95 CI 7·10-7·59, p<0·0001). Infants of women who gained more than 24 kg during pregnancy were 148·9 g (141·7-156·0) heavier at birth than were infants of women who gained 8-10 kg. The odds ratio of giving birth to an infant weighing more than 4000 g was 2·26 (2·09-2·44) for women who gained more than 24 kg during pregnancy compared with women who gained 8-10 kg. Interpretation Maternal weight gain during pregnancy increases birthweight independently of genetic factors. In view of the apparent association between birthweight and adult weight, obesity prevention efforts targeted at women during pregnancy might be beneficial for offspring. Funding US National Institutes of Health.
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