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 abru ption in first and second pregnancies and to compare the risk-factor profil es for evidence of etiologic heterogeneity. A prospective cohort design was used. The study took place at university-based medical centers that partic ipated in the U.S. Collaborative Perinatal Project (1959-1965). A total of 10,774 first pregnancies only and 6529 first and second pregnancies of wome n were enrolled in the study. Participation rate was 96%. All pregnancies w ere selected at some centers, whereas other centers used either random or s ystematic sampling. The main outcome measure was placental abruptions in fi rst and second pregnancies. The placental abruption rate was 1.7% (n = 182) for first and 2.2% (n = 143) for second pregnancies. Prior abruption incre ased risk in second pregnancies significantly (odds ratio [OR] = 3.2, 95% c onfidence interval [CI]: 1.7-5.8) after adjusting for other risk factors. P lacental infarcts and smoking duration were associated with an increased ri sk for abruption in second but not first pregnancies. Effect of placental i nfarcts was modified by gestational age with strongest risk for abruption a t 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 placenta l abruption for first and second pregnancies were different, indicating het erogeneity in their risk-factor profiles. Future research would best consid er abruption as a heterogeneous complication to further knowledge of its et iology. J CLIN EPIDEMIOL 52;5:453-461, 1999. (C) 1999 Elsevier Science Inc.