Objective: To examine the association between body mass index (BMI) at 11–13 weeks’ gestation and a wide range of adverse pregnancy outcomes after adjustment for confounding factors in obstetric history and maternal characteristics. Methods: This was a prospective screening study for adverse obstetric outcomes in women with singleton pregnancies attending for their first routine hospital visit at 11+0–13+6 weeks of gestation. The maternal weight and height were measured and the BMI was calculated. Regression analysis was performed to examine the association between BMI and each of the adverse pregnancy outcomes. Results: We examined 41,577 pregnancies with a live fetus at 11–13 weeks. There was a significant contribution from maternal BMI, in addition to maternal characteristics and obstetric history, in the prediction of subsequent miscarriage, stillbirth, preeclampsia, gestational hypertension, gestational diabetes mellitus, delivery of small and large for gestational age neonates, and both elective and emergency cesarean section, but not spontaneous preterm delivery. The risk for each pregnancy complication increased exponentially with BMI, except for delivery of small for gestational age neonates which decreased with BMI. Conclusions: Maternal BMI at 11–13 weeks can be combined with other maternal characteristics and obstetric history to estimate patient-specific risks for many pregnancy complications.