This systematic review focuses on outcomes of gestational weight gain, specifically birthweight, fetal growth, and postpartum weight retention, for singleton pregnancies with respect to the 1990 Institute of Medicine weight gain recommendations. A total of 35 studies met the inclusion criteria and were reviewed. There was strong evidence to support associations between excessive gestational weight gain and increased birthweight and fetal growth (large for gestational age) as well as inadequate gestational weight gain and decreased birthweight and fetal growth (small for gestational age). There was moderate evidence to support the association between excessive gestational weight gain and postpartum weight retention. Clear clinical recommendations based on this review are challenging because of several limitations in the literature. Improvements in future research include the use of consistent definitions of gestational weight gain and outcomes of interest, assessment of confounders, and better collection of weight and weight gain data.