Objective To examine whether delivery by caesarean section is a risk factor for childhood obesity.

Design Prospective prebirth cohort study (Project Viva).

Setting Eight outpatient multi-specialty practices based in the Boston, Massachusetts area.

Participants We recruited women during early pregnancy between 1999 and 2002, and followed their children after birth. We included 1255 children with body composition measured at 3 years of age.

Main outcome measures BMI score, obesity (BMI for age and sex ≥95th percentile), and sum of triceps plus subscapular skinfold thicknesses at 3 years of age.

Results 284 children (22.6%) were delivered by caesarean section. At age 3, 15.7% of children delivered by caesarean section were obese compared with 7.5% of children born vaginally. In multivariable logistic and linear regression models adjusting for maternal prepregnancy BMI, birth weight, and other covariates, birth by caesarean section was associated with a higher odds of obesity at age 3 (OR 2.10, 95% CI 1.36 to 3.23), higher mean BMI z-score (0.20 units, 95% CI 0.07 to 0.33), and higher sum of triceps plus subscapular skinfold thicknesses (0.94 mm, 95% CI 0.36 to 1.51).

Conclusions Infants delivered by caesarean section may be at increased risk of childhood obesity. Further studies are needed to confirm our findings and to explore mechanisms underlying this association.