Introduction Obesity is worldwide epidemic and increase in cesarean section rates have occurred in parallel. Overweight children had a lower proportion of the genus Bifidobacterium spp. in their intestinal microflora during infancy. Infants born by cesarean section have less Bifidobacterium spp. as predominant microbiota.

Objective we hypothesised that infants born by cesarean section are more likely to develop obesity in adulthood.

Methods We carried out a newborn cohort study in Ribeirão Preto, Brazil, started in 1978. A randomised sample of 2057 subjects from the original cohort (6827 individuals) was reassessed in 2002. Some co-variables were collected after birth: type of delivery, birth weight, maternal smoking and maternal schooling. The data from subjects were obtained at the time of their return for evaluation at 24 years of age: body mass index (BMI), physical activity, subject smoking, and income in minimum wages. Obesity was considered when BMI≥30. A Poisson multivariable model was performed aiming to determine the impact of cesarean section on BMI at adulthood. The model was adjusted for subject and maternal factors.

Results The rates of obesity in young adults born by cesarean section was 15.2% vs 10.4% in those born by vaginal delivery (p=0.002) Subjects who were born by cesarean section had an increased significant risk [1.57 (1.23–2.02)] for obesity at adulthood after controlling.

Conclusion We may hypothesise that the differences in intestinal flora related to type of delivery section may have a role on the epidemic obesity worldwide.