Impact of maternal age on the incidence of obstetrical complications in Japan.

To clarify the effect of maternal age on obstetrical complications through a cohort and case-cohort study. We studied 242 715 births at 125 centers of a perinatal network in Japan from 2001 through 2005 as a base cohort. Women with single pregnancies who delivered after 22 weeks of gestation were included in the study. Six classes of maternal age were selected: <20; 20-24; 25-29; 30-34; 35-39; and ≥40 years. The cohort study was used to investigate whether age is related to obstetrical complications. By random selection 3749 births were determined as a subcohort. Risk ratio (RR) was determined using multivariate analysis in the case-cohort study. The incidence proportion (per 100 births) of pregnancy-induced hypertension, cervical insufficiency, placenta previa, and placental abruption increased with age, whereas the incidence proportion of preterm labor and chorioamnionitis were higher at younger maternal age. The RR of women in the age groups 35-39 years and ≥40 years (with the reference of 1.0 for women in the age group of 20-34 years) were determined: pregnancy-induced hypertension, 1.66, 2.55; placenta previa, 1.76, 2.19; and placental abruption, 1.18, 1.5. The RR of preterm labor for women in the age group of <20 years was 1.78. The effect of maternal age differs for each obstetrical complication, and thus, it is important to understand these differences for management of individual pregnant patients.