Statistical analyses
The data of the women with placental abruption were reviewed. The maternal background and obstetric outcomes were compared between women with and without placental abruption. We also examined the frequency of placental abruption according to the gestational age. The chi-square test and Fisher’s exact test were used to compare the categorical variables, and t-test was used to compare the continuous variables after confirming that each of the continuous variables was normally distributed. The adjusted OR (aOR) and 95% CI for placental abruption were calculated using the multiple logistic regression model. The ORs were adjusted for confounding variables, including history of placental abruption, ART pregnancy, parity, maternal age, smoking during pregnancy, BMI before pregnancy, chronic hypertension, and uterine myoma. In the logistic regression model, dummy variables were used for the categorical variables that consisted of more than three categories. All statistical analyses were conducted using SPSS, version 26 (IBM Corp., Armonk, NY, USA). The differences with P <0.05 were considered statistically significant.