OBJECTIVE: To evaluate whether morbidly obese women have an increased risk of pregnancy complications and adverse perinatal outcomes. METHODS: In a prospective population-based cohort study, 3,480 women with morbid obesity, defined as a body mass index (BMI) more than 40, and 12,698 women with a BMI between 35.1 and 40 were compared with normal-weight women (BMI 19.8-26). The perinatal outcome of singletons born to women without insulin-dependent diabetes mellitus was evaluated after suitable adjustments. RESULTS: In the group of morbidly obese mothers (BMI greater than. 40) as compared with the normal-weight mothers, there was an increased risk of the following outcomes (adjusted odds ratio, 95% confidence interval): preeclampsia (4.82, 4.04, 5.74), antepartum stillbirth (2.79, 1.94,4.02), cesarean delivery (2.69, 2.49,2.90), instrumental delivery (1.34, 1.16, 1.56), shoulder dystocia (3.14, 1.86, 5.31), meconium aspiration (2.85, 1.60, 5.07), fetal distress (2.52, 2.12,2.99), early neonatal death (3.41, 2.07,5.63), and large-for-gestational age (3.82, 3.50, 4.16). The associations were similar for women with BMIs between 35.1 and 40 but to a lesser degree. CONCLUSION: Maternal morbid obesity in early pregnancy is strongly associated with a number of pregnancy complications and perinatal conditions. tricians and Gynecologists.