Comparison of risk factors for major obstetric complications between Western countries and Japan: a case-cohort study.

The aim of this study was to demonstrate the differences in risk factors for obstetrical complications between Japan and Western countries. Using the Perinatal Database of the Japan Society for Obstetrics and Gynecology, we studied singleton deliveries after 22 weeks of gestation (n = 242 715) at 125 centers of the perinatal network in Japan from 2001 through 2005 as a base cohort. In total, 3749 births (1.5% of the base cohort) were randomly selected as a subcohort. We compared the rate of risk factors in the cases with ten obstetrical complications with that in the subcohort (case-cohort study). Almost all of the evaluated risk factors were common between Western countries and Japan. Older age at pregnancy was a common risk factor for pregnancy-induced hypertension, placental abruption, placenta previa, and placenta accreta/increta/percreta. On the other hand, younger age at pregnancy was a common risk factor for eclampsia and preterm delivery. Smoking during pregnancy was a common risk factor for pregnancy-induced hypertension, preterm premature rupture of the membranes, preterm delivery, cervical insufficiency, chorioamnionitis, and placental abruption. In vitro fertilization and embryo transfer was a common risk factor for cervical insufficiency, placenta previa, and placenta accreta/increta/percreta. This case-cohort study in Japan clarified the common risk factors between Western countries and Japan as well as the risk factors indigenous to Japanese women. To identify the risk factors for a disease in a specific country, we should use data derived from its population.