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Notes: Objective  To show the increased risk of adverse outcomes in labour and fetomaternal morbidity in obese women (BMI 〉 30).Design  A population-based observational study.Setting  University Hospital of Wales. The study sample was drawn from the Cardiff Births Survey, a population-based database comprising of a total of 60,167 deliveries in the South Glamorgan area between 1990 and 1999.Population  Primigravid women with a singleton uncomplicated pregnancy with cephalic presentation of 37 or more weeks of gestation with accurate information regarding height and weight recorded at the booking visit (measured by the midwives) were included in the study.Methods  Comparisons were made between women with a body mass index of 20–30 and those with more than 30. SPSS version 10 was used for statistical analysis. Student's t test, χ2 and Fisher's exact tests were used wherever appropriate.Main outcome measures  Labour outcomes assessed were risk of postdates, induction of labour, mode of delivery, failed instrumental delivery, macrosomia and shoulder dystocia. Maternal adverse outcomes assessed were postpartum haemorrhage, blood transfusion, uterine and wound infection, urinary tract infection, evacuation of uterus, thromboembolism and third- or fourth-degree perineal tears. Fetal wellbeing was assessed using Apgar 〈7 at 5 minutes, trauma and asphyxia, cord pH 〈 7.2, babies requiring neonatal ward admissions, tube feeding and incubator.Results  We report an increased risk [quoted as odds ratio (OR) and confidence intervals CI)] of postdates, 1.4 (1.2–1.7); induction of labour, 1.6 (1.3–1.9); caesarean section, 1.6 (1.4–2); macrosomia, 2.1 (1.6–2.6); shoulder dystocia, 2.9 (1.4–5.8); failed instrumental delivery, 1.75 (1.1–2.9); increased maternal complications such as blood loss of more than 500 mL, 1.5 (1.2–1.8); urinary tract infections, 1.9 (1.1–3.4); and increased neonatal admissions with complications such as neonatal trauma, feeding difficulties and incubator requirement.Conclusion  Obese women appear to be at risk of intrapartum and postpartum complications. Induction of labour appears to be the starting point in the cascade of events. They should be considered as high risk and counselled accordingly.