Explaining the association of maternal age with cesarean delivery for nulliparous and parous women

We used data on 6,805 live births from the National Maternal and Infant Health Survey to examine the positive association between maternal age and Cesarean delivery. For parous women, the unadjusted risk ratio [RR] for Cesarean, comparing 35 year olds with 20 year olds, was estimated to be 1.40 [95% confidence limits (CL) = 1.18, 1.65]; this association disappeared upon adjustment for obstetrical predictors and other factors (adjusted RR = 1.04; 95% CL = 0.80, 1.35). In contrast, for nulliparous women, the unadjusted risk ratio was 2.16 (95% CL = 1.78, 2.63) and remained substantial upon adjustment (RR = 1.74; 95% CL = 1.25, 2.43). These results, along with baseline risk information, suggest that older nulliparous women and their obstetricians should be the target of future efforts to control Cesarean rates.