Impact of labor induction, gestational age, and maternal age on cesarean delivery rates.

OBJECTIVE To quantify the impact of labor induction and maternal age on cesarean delivery rates in nulliparous and multiparous women between 36 and 42 weeks' gestation. METHODS We performed a retrospective cohort study on 14,409 women delivering at two teaching hospitals in metropolitan Boston during 1998 and 1999. Women who had contraindications to labor, including a prior cesarean delivery, were excluded. The risks for cesarean delivery by induction status, gestational age by completed week between 36 and 42 weeks, maternal age <35, 35-39, and >/=40 years, and stratified by parity, were calculated by logistic regression. RESULTS In nulliparas, labor induction was associated with an increase in cesarean delivery from 13.7% to 24.7% (adjusted odds ratio [OR] 1.70; 95% confidence interval [CI] 1.48, 1.95]). In multiparas, induction was associated with an increase from 2.4% to 4.5% (OR 1.49; 95% CI 1.10, 2.00). Other variables that placed a nulliparous woman at increased risk for cesarean delivery included maternal age of at least 35 years and gestational ages over 40 weeks. For multiparas, only maternal age 40 years or older and gestational age of 41 weeks were associated with an increase in cesarean deliveries. CONCLUSION Induction of labor, older maternal age, and gestational age over 40 weeks each independently increase the risk for cesarean delivery in both nulliparous and multiparous women. Although the relative risk from induction is similar in nulliparas and multiparas, the absolute magnitude of the increase is much greater in nulliparas (11% versus 2.1%).