Pre‐Pregnancy and Early‐Pregnancy Risk Factors for Placental Abruption: A Nationwide Retrospective Cohort Study in Japan

Aim To identify pre‐ and early‐pregnancy risk factors for placental abruption using a nationwide perinatal registry in Japan. Methods We evaluated 1 706 721 singleton pregnancies delivered at ≥ 22 weeks' gestation during 2014–2023 in the Japan Society of Obstetrics and Gynecology perinatal registry. Primary analysis: multivariable binary logistic regression estimated adjusted odds ratios (aORs) with 95% confidence intervals for placental abruption, adjusting for 56 prespecified baseline covariates. Secondary analysis: multinomial logistic regression accounted for preterm birth as a competing pregnancy outcome. Exploratory analyses: a conditional inference tree explored risk‐factor combinations; multinomial and within‐case binary logistic regression compared abruption with and without intrauterine fetal death (IUFD). Results Placental abruption occurred in 16 016 pregnancies (0.94%). History of placental abruption (aOR 5.80) and hypertensive disorders of pregnancy (aOR 1.54), pre‐existing essential hypertension (aOR 1.77), advanced maternal age, low pre‐pregnancy body mass index (BMI), smoking during pregnancy, and velamentous (aOR 2.36) and marginal (aOR 1.76) cord insertions were independently associated with increased odds of abruption; associations were largely consistent in the multinomial analysis, and abnormal umbilical cord insertion was the primary splitting variable. Established risk factors were shared between abruption with and without IUFD, whereas smoking, higher BMI, and previous cesarean delivery were more strongly associated with the IUFD subtype. Conclusions Established maternal vascular and obstetric risk factors were associated with placental abruption and largely shared across subtypes defined by IUFD status. Abnormal umbilical cord insertion was also associated with abruption, suggesting that abnormal placental development may be related to the pathogenesis of this condition.