To compare rates of Caesarean section between mothers of advanced age (35 to 40, and over 40 years) and those aged 20 to 34, using the Robson classification system to examine additional maternal factors.

A total of 134 088 hospital deliveries in Ontario between April 1, 2011, and March 31, 2012, were grouped into Robson's 10 mutually exclusive and totally inclusive classification categories. Records from the three Robson groups that made the greatest contribution to the overall CS rate were stratified by maternal age, health condition, obstetrical complication, assisted reproductive technology usage, smoking during pregnancy, and socioeconomic status.

Rates of CS increased with advancing maternal age; in women aged 20 to 34, 35 to 40, and over 40, the rates were 26.2%, 35.9%, and 43.1%, respectively. The top three Robson groups by contribution to CS rates involved women who had one or more of the following factors: previous Caesarean section, primiparity, conception by means of assisted reproductive technology, chronic hypertension, gestational diabetes, diabetes mellitus, preeclampsia, placenta previa, placental abruption, or large for gestational age infants. The prevalence of these factors increased with advancing maternal age, yet mothers aged ≥ 35 with one or more health conditions or obstetrical complications had higher CS rates than mothers aged 20 to 34 with the same condition(s) or complication(s).

Health conditions and obstetrical complications alone in older women do not account for increased rates of CS. The preferences of the individual care provider and the mother on CS rates may play a key role and require further investigation.