Delivery by caesarean section and childhood cancer: a nationwide follow-up study in three countries.

POPULATION: Children born in Denmark (1973-2007), Sweden (1973-2006) and Finland (randomly selected sample of 90%, 1987-2007; n = 7,029,843). METHODS: Exposure was delivery by caesarean section and the outcome was childhood cancer diagnosis. Follow-up started from birth and ended at the first of the following dates: cancer diagnosis, death, emigration, day before 15th birthday or end of follow-up. Cox regression was used to obtain hazard ratios. MAIN OUTCOME MEASURES: Childhood cancer diagnosis. RESULTS: A total of 882,907 (12.6%) children were delivered by caesarean section. Of these, 30.3% were elective (n = 267,603), 35.9% unplanned (n = 316,536) and 33.8% had no information on planning (n = 298,768). Altogether, 11,181 children received a cancer diagnosis. No evidence of an increased risk of childhood cancer was found for children born by caesarean section (hazard ratio, 1.05; 95% confidence interval, 0.99, 1.11). No association was found for any major type of childhood cancer, or when split by the type of caesarean section (elective/unplanned). CONCLUSION: The evidence does not suggest that caesarean section is a risk factor for the overall risk of childhood cancer and possibly not for subtypes of childhood cancer either.