Background National guidelines advocate that obese pregnant women be made aware of their increased risks during pregnancy. This study pooled a range of high-quality data to estimate the absolute and attributable risks of delivery by caesarean section (CS) for obese women, and to model the effect of pre-pregnancy weight loss.

Methods Estimates of the body mass index profile of the maternal population and the proportion of deliveries by CS were obtained from nationally representative sources. The relative risk of delivery by CS was estimated by pooling published data. These were equated to estimate absolute risks, attributable risks and numbers needed to prevent one CS, based on 10% pre-pregnancy weight loss.

Results The relative risk of CS for an obese pregnant woman, compared women of recommended weight is 1.73 (95% CI 1.61 to 1.87). The estimated probability of delivery by CS for an obese pregnant woman is 35.3% (95% CI 33.2 to 37.4), compared to 20.3% (20.0–20.7) for women of recommended weight. An estimated 8.4% (7.9–9.0) of all CS deliveries in England are attributable to maternal obesity. A reduction of 10% in pre-pregnancy weight for an obese woman is associated with an estimated reduction of 14% (12–15) in their probability of delivery by CS. 20 (17–24) obese women need to lose 10% of their body weight to prevent one CS delivery.

Conclusions This study provides estimates of an individual's probability of, and the population burden of, delivery by CS due to maternal obesity. These results have implications for assessing the cost-effectiveness of pre-pregnancy weight loss programmes.