The objective of the study was to investigate the association between increasing maternal body mass index (BMI) and elective/emergency caesarean delivery rates. Systematic review and meta-analysis of published cohort studies were used. The bibliographic databases, MEDLINE, EMBASE, CINAHL, were searched systematically, with no language restrictions, from 1996 to May 2007. MeSH terms and key words for 'pregnancy', 'obesity', 'overweight,''body mass index' and 'caesarean section' were combined with the Cochrane Collaboration strategy for identifying primary studies. Finally, 11 papers were considered eligible for inclusion in the review. Although all the papers were cohort studies, only three were prospective in nature. Compared with women with normal BMI (20-25 kg m(-2)), the crude pooled odds ratios (95% confidence intervals) for caesarean section in overweight (BMI 25-30 kg m(-2)), obese (BMI 30-35 kg m(-2)) and morbidly obese (BMI > 35 kg m(-2)) women were 1.53 (1.48, 1.58), 2.26 (2.04, 2.51) and 3.38 (2.49, 4.57) respectively. The pooled odds of having an emergency caesarean section were 1.64 (95% confidence intervals 1.55, 1.73) in overweight and 2.23 (2.07, 2.42) in obese women. Caesarean delivery risk is increased by 50% in overweight women and is more than double for obese women compared with women with normal BMI.