Obesity is a strong determinant of obstetric morbidity and mortality. This chapter summarizes the current state of knowledge of the relationship between obesity and adverse maternal outcome, including effects on the health of the pregnant mother as well as complications at the time of delivery. Total body fat can be measured by direct methods such as dual energy X-ray absorptiometry (DEXA) and magnetic resonance imaging (MRI). Maternal obesity is overrepresented in maternal deaths in developed countries. Obese pregnant women have an increased incidence of labor induction. The evidence shows a marked increased risk of adverse obstetric events in obese pregnancies, including gestational diabetes, hypertensive diseases, thromboembolism, infection, cesarean section, and postpartum hemorrhage. There are also clear indications that risks increase as body mass index (BMI), or the levels of obesity, increase. In most developed countries the prevalence of obesity is around 20%.