Risk factor profiles of placental abruption in first and second pregnancies: heterogeneous etiologies.

The objective of this study was to identify risk factors for placental abruption in first and second pregnancies and to compare the risk-factor profiles for evidence of etiologic heterogeneity. A prospective cohort design was used. The study took place at university-based medical centers that participated in the U.S. Collaborative Perinatal Project (1959-1965). A total of 10,774 first pregnancies only and 6529 first and second pregnancies of women were enrolled in the study. Participation rate was 96%. All pregnancies were selected at some centers, whereas other centers used either random or systematic sampling. The main outcome measure was placental abruptions in first and second pregnancies. The placental abruption rate was 1.7% (n = 182) for first and 2.2% (n = 143) for second pregnancies. Prior abruption increased risk in second pregnancies significantly (odds ratio [OR] = 3.2, 95% confidence interval [CI]: 1.7-5.8) after adjusting for other risk factors. Placental infarcts and smoking duration were associated with an increased risk for abruption in second but not first pregnancies. Effect of placental infarcts was modified by gestational age with strongest risk for abruption at shortest gestations. For each year of smoking prior to pregnancy, risk of abruption increased 40% (OR = 1.4, 95% CI 1.0-1.8). Etiologies of placental abruption for first and second pregnancies were different, indicating heterogeneity in their risk-factor profiles. Future research would best consider abruption as a heterogeneous complication to further knowledge of its etiology.