Caesarean section among relatives.

BACKGROUND Caesarean section rates are increasing. It is not clear whether a familial predisposition to caesarean section exists. METHODS Births registered in the Medical Birth Registry of Norway during 1967-2005 were linked to construct 440,236 grandmother-parent units and 275,001 same sex full sibling units. Log-binomial regression models were used to explore associations. RESULTS A mother born by caesarean section had a 55% increased risk of having her first child by caesarean section [adjusted relative risk (RR) 1.55, 95% CI 1.48-1.62]. The same was not found for fathers born by caesarean section (RR 1.02, 95% CI 0.96-1.10). A younger sister, whose older sister had her first child by caesarean section, had a 45% increased risk of having her first child by caesarean section (RR 1.45, 95% CI 1.40-1.51). If an older brother had his first child by caesarean, this did not constitute an increased risk for the younger brother's partner (RR 1.02, 95% CI 0.97-1.08). Similar associations were found in the subgroups of obstetric low risk deliveries. CONCLUSIONS A female-to-female familial predisposition to caesarean section was observed. It could be caused by biologic inheritance, primarily working through maternal alleles and/or environmental factors. The results imply that both mechanisms could be important.