While the National Academy of Medicine, formerly known as the Institute of Medicine, recommends the optimum weight change in the second and third trimesters combined, the ideal weight change range per week according to trimesters remains unexplored.

To determine the optimal rates of weight change per week that are associated with decreased adverse pregnancy outcomes for each trimester stratified by prepregnancy body mass index categories.

This was a secondary analysis of a prospective cohort study using data from the Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-to-Be (2010–2014). Our exposure was the weight change per week, defined as weight change (kg) divided by gestational weeks interval. We examined weight change per week for each trimester: weight change in early pregnancy (early first trimester to second trimester); second trimester (second trimester to third trimester); and third trimester (third trimester to delivery). Our primary outcome was a composite of adverse pregnancy outcomes including hypertensive disorders of pregnancy, preterm birth, spontaneous preterm birth, small-for-gestational-age birth, and stillbirth. To explore potential nonlinear associations between pregnancy weight change and adverse pregnancy outcomes, we applied restricted cubic spline functions with 5 knots. We calculated adjusted relative risks with 95% confidence intervals using generalized linear models with Poisson distribution and robust error variance, controlling for confounders. We estimated adjusted relative risks of adverse pregnancy outcomes for each weight change per week and identified the optimal weight change. The optimal weight change was compared with the National Academy of Medicine–recommended weight change in pregnancy.

Of the 8121 patients included, 4373 (53.8%) had a normal or underweight body mass index, 2002 (24.7%) had an overweight body mass index, and 1746 (21.5%) had an obese body mass index. The weight changes per week associated with the lowest adverse risks for individuals with normal/underweight, overweight, and obese body mass index were 0.7 kg/wk, 0.7 kg/wk, and 0.6 kg/wk, respectively, for the first trimester, 0.7 kg/wk, 0.5 kg/wk, and 0.5 kg/wk, respectively, for the second trimester, and 0.5 kg/wk, 0.4 kg/wk, and 0.4 kg/wk, respectively, for the third trimester. For normal/underweight, weight gain according to the National Academy of Medicine guidelines in the second trimester compared to our optimal weight gain was associated with an increased risk of adverse pregnancy outcomes and preterm birth. For normal/underweight and overweight individuals in the second trimester, weight gain according to the National Academy of Medicine guidelines, compared to our optimal weight gain, was associated with an increased risk of small for gestational age. Conversely, for individuals with obesity in the third trimester, weight gain according to the National Academy of Medicine guidelines compared to our optimal weight gain was associated with a decreased risk of cesarean delivery.

The gestational weight change rates associated with the lowest adverse risks were of higher values than those established by the National Academy of Medicine, which may suggest that the National Academy of Medicine guidelines are too strict.