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OBJECTIVE: To examine the maternal and foetal risks of adverse pregnancy outcome in relation to maternal obesity, expressed as body mass index (BMI, kg=m 2 ) in a large unselected geographical population. DESIGN: Retrospective analysis of data from a validated maternity database system which includes all but one of the maternity units in the North West Thames Region. A comparison of pregnancy outcomes was made on the basis of maternal BMI at booking. SUBJECTS: A total of 287 213 completed singleton pregnancies were studied including 176 923 (61.6%) normal weight (BMI 20 -24.9), 79 014 (27.5%) moderately obese (BMI 25 -29.9) and 31 276 (10.9%) very obese (BMI ! 30) women. MEASUREMENTS: Ante-natal complications, intervention in labour, maternal morbidity and neonatal outcome were examined and data presented as raw frequencies and adjusted odds ratios with 99% confidence intervals following logistic regression analysis to account for confounding variables. RESULTS: Compared to women with normal BMI, the following outcomes were significantly more common in obese pregnant women (odds ratio (99% confidence interval) for BMI 25 -30 and BMI ! 30 respectively): gestational diabetes mellitus (delivery before 32 weeks' gestation (0.73 (0.65 -0.82), 0.81 (0.69 -0.95)) and breastfeeding at discharge (0.86 (0.84 -0.88), 0.58 (0.56 -0.60)) were significantly less likely in the overweight groups. In all cases, increasing maternal BMI was associated with increased magnitude of risk. CONCLUSION: Maternal obesity carries significant risks for the mother and foetus. The risk increases with the degree of obesity and persists after accounting for other confounding demographic factors. The basis of many of the complications is likely to be related to the altered metabolic state associated with morbid obesity.