Relationships between body mass index (BMI) and weight gain with perinatal outcome and birthweight were examined. BMI was calculated on 582 consecutive pregnant women who delivered at or >37 weeks gestational age. Statistical analysis was done using Chi-square tests, analysis of variance, and multiple logistic regression. Of those studied, 13% were underweight, 39% normal, 13% overweight, and 35% obese. Obesity was associated with increasing age (P < .01), multiparity (P < .01), previous cesarean delivery (P < .01), previous macrosomia (P = .01), previous fetal death (P = .03), hypertensive disorders (P < .01), gestational diabetes (P = .02), cesarean delivery (P = .03), and neonatal intensive care unit admission (NICU) (P = .01). The underweight group had the most low birthweight (LBW) infants and the lowest mean birthweight. Ideal weight gain occurred in 31%, inadequate weight gain in 34%, and excessive weight gain in 35%. Inadequate weight gain had increased asthma (P < .05), and hyperemesis (P = .03). Women with ideal weight gain had less smokers (P < .01), fetal distress (P < .05), cesarean delivery (P = .02), and preeclampsia (P < .001). The mean birthweight was highest in the excessive weight gain (P < .01). With multivariate analysis, previous LBW, BMI, and tobacco use were significant predictors of LBW. Normal BMI and ideal weight gain in pregnancy is associated with decreased perinatal complications and an optimum birthweight.