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To determine whether the association between maternal weight gain during pregnancy and neonatal weight is independent of genetic factors, researchers conducted a cohort study using birth registry data from more than 1 million singleton term births to more than 500,000 mothers in Michigan and New Jersey from 1989 through 2003. A within-subject study design (in which outcomes of sequential pregnancies were evaluated in each mother) allowed the authors to control for genetic confounders.
Many women exceeded the recommended limits for weight gain during a singleton pregnancy. Relative to a weight gain of 8–10 kg (reference category), mothers who gained 20–22 kg had infants who weighed a mean of 103.8 g more. Mothers who gained >24 kg had infants who weighed a mean of 148.9 grams more. Overall, for each kilogram increase in maternal weight gain, birthweight rose by 7.35 g.
Ludwig DS and Currie J. The association between pregnancy weight gain and birthweight: A within-family comparison. Lancet 2010 Aug 5; [e-pub ahead of print]. (http://dx.doi.org/10.1016/S0140-6736(10)60751-9)
Comment
Although the authors were unable to determine prepregnancy body-mass index, the large sample size and within-subject design of this study add credence to the results, which show that when a mother gains excessive weight during pregnancy — independent of her genetic makeup — her baby also will weigh more. The ramifications of this finding extend beyond childbirth to the lifelong health of the child (e.g., obesity, diabetes) and should prompt us to provide emphatic preconceptual counseling about pregnancy weight gain and to alert mothers to the Institute of Medicine guidelines (JW Womens Health Jul 2 2009).