Caesarean birth in women with infertility: population-based cohort study.

OBJECTIVE Caesarean section (CS) is more common following infertility treatment (IT), but the reasons why remain unclear and confounded. The Robson 10-Group Classification System (TGCS) may further explain variation in CS rates. We assessed the association between mode of conception and CS across Robson groups. DESIGN Population-based cohort study. SETTING Ontario, Canada, in a public healthcare system. POPULATION 921,023 births, 2006-2014. METHODS Modified Poisson regression produced relative risks (RR) and 95% confidence intervals, comparing the risk of CS among women with (i) subfertility without IT, (ii) non-invasive IT (OI, IUI), or (iii) invasive IT (IVF) - each relative to (iv) spontaneous conception (SC). MAIN OUTCOME MEASURES CS rate according to one of four modes of conception, overall, and stratified by each of the TGCS groups. RESULTS Relative to SC (26.9%), the risk of CS increased in those with subfertility without IT (RR 1.17; 1.16-1.18), non-invasive IT (RR 1.21; 1.18-1.24), and invasive IT (RR 1.39; 1.36-1.42). Within each Robson group, similar patterns of RRs were seen, but with markedly differing rates. For example, in Group 1 (nulliparous, singleton, cephalic at ≥ 37 weeks, with spontaneous labour), the respective rates were 15.0%, 19.4%, 18.7%, and 21.9%. In Group 2 (nulliparous, singleton, cephalic at ≥ 37 weeks, without spontaneous labour), the rates were 35.9%, 44.4%, 43.2%, and 54.1%, while in Group 8 (multiple pregnancy), they were 55.9%, 67.5%, 65.0%, and 69.3%, respectively. CONCLUSIONS CS is relatively more common in women with subfertility and those receiving IT - an effect that persists across Robson groups.