Background The Institute of Medicine (IOM) has recently made new recommendations for total and trimester gestational weight gain. However, deviates from recommended weight gain according to gestational trimester have been little investigated with regard to obstetric outcomes.

Objectives To investigate association between weekly weight gain during second and third trimesters, and maternal and fetal outcomes, taking into account pregestational body mass index and other variables.

Methods Gestational weight gain was evaluated in 3063 pregnant women of the Brazilian Study on Gestational Diabetes (Estudo Brasileiro do Diabetes Gestacional—EBDG) and classified according to the last IOM recommendation. Outcomes were cesarean delivery, preterm birth and small or large for gestational age. Associations between adequate weight gain and outcomes were estimated using Poisson regression. Possible confounders were pregestational body mass index, age, height, race, parity, education, smoking, alcohol consumption, diabetes and hypertensive disorders.

Results In fully adjusted models, second trimester insufficient weight gain was associated with small for gestational age (RR 1.55 95% CI 1.19 to 2.01); second trimester excessive weight gain with large for gestational age (RR 1.58 95% CI 1.20 to 2.08); third trimester insufficient weight gain with preterm birth (RR 1.55 95% CI 1.08 to 2.22). Third trimester excessive weight gain was associated with preterm birth (RR 1.54 95% CI 1.07 to 2.21) and cesarean delivery (RR 1.17 95% CI 1.04 to 1.33).

Conclusion Deviates from recommended weight gain during each trimester are associated with adverse pregnancy outcomes. These findings support the IOM recommendations for nutritional care during pregnancy.