The Institute of Medicine (IOM) recently published new guidelines for maternal weight gain during pregnancy. Using data collected at Moffitt Hospital, University of California at San Francisco, we examined the associations between maternal weight gain outside the recommendations of the IOM and three pregnancy outcomes (small for gestational age [SGA] infants, large for gestational age [LGA] infants, and cesarean delivery). These analyses were repeated using population-specific weight gain ranges derived from a subgroup of women in this cohort with healthy pregnancy outcomes. The purpose of the study was to compare the national guidelines of the IOM with hospital standards. Both the recommendations of the IOM and the hospital ranges were associated with fewer SGA infants, LGA infants, and cesarean deliveries. The overall associations were similar for the population-derived and national ranges. Although the IOM ranges should be examined in other populations and with other birth outcomes, our results validated their recommendations. Maternal weight gain within the IOM recommendations reduced the risk of the outcomes studied; furthermore, these ranges performed as well as those derived from the target population.