To assess pregnancy outcomes in different BMI groups. We analyzed 25,601 singleton pregnancies from January 1989 to December 2001. Overweight women (prepregnancy BMI = 26 to 29 kg/m2) represented 13.2% (3388) of the cases, and 7.3% (1880) were obese (BMI > or = 30 kg/m2). The data were obtained from self-administered questionnaires at 20 weeks of pregnancy, complemented by nurse interviews and clinical records. Multiple logistic regression analysis was used to control for confounding factors. Overweight and obese women had more previous deliveries, pregnancy terminations, miscarriages, and stillbirths, to have more diabetes and hypertension, and to smoke more often than normal weight women. The pregnancies were more often complicated by preeclampsia or chorioamnionitis (p < 0.001). Pregnancy outcomes were impaired in overweight and obese pregnant women, with respective odds ratios (95% confidence index) as follows: low Apgar score at 5 minutes, 1.54 (1.20 to 1.98) and 1.64 (1.22 to 2.28); newborn admission to a neonatal unit, 1.20 (1.06 to 1.37) and 1.38 (1.17 to 1.61); cesarean delivery, 1.22 (1.10 to 1.35) and 1.68 (1.48 to 1.91); fetal death, 1.54 (0.88 to 2.68) and 2.35 (1.28 to 4.32); perinatal death, 1.54 (0.98 to 2.42) and 2.19 (1.33 to 3.62). Obesity, in particular during pregnancy, should be considered as an abnormal situation. An overweight condition increases obstetric risks in a BMI-dependent manner. The risk of perinatal death more than doubles in the transition from an overweight to an obese condition. Modest weight loss could bring substantial advantages to obstetric outcome.