Abstract
Objective. To evaluate secular trends in the occurrence of placenta previa and whether placenta previa is associated with the outcome of previous pregnancies, cesarean section, and sociodemographic factors. Design. A cohort study based on the Medical Birth Registry of Norway. Placenta previa in the second pregnancy was investigated for associations with outcomes in the first pregnancy and sociodemographic factors. Results. In birth orders 1 and 2 the occurrence of placenta previa was 1.2 and 2.2 per 1000, respectively, with no secular trend. The occurrence increased with maternal age and was lowest in women aged 20-29 years. The recurrence rate was 23 per 1000 (adjusted odds ratio (OR) of recurrence=9.7). In women with prior delivery at ≤25 gestational weeks the risk of placenta previa was 6.7 per 1000 (adjusted OR=3.0). In women with prior placental abruption the risk was 5.8 per 1000 (OR=2.6). In women with prior perinatal death the risk was 4.4 per 1000 (adjusted OR=1.8). No independent relationship emerged with socio-economic factors, previous birthweight, and a history of pregnancy induced hypertension. Cesarean section was associated with subsequent development of placenta previa (adjusted OR=1.3). Conclusions. We found no secular trends in the occurrence of placenta previa. Placenta previa is associated with previously described risk factors for placental abruption. The increased risk of placenta previa subsequent to placental abruption supports the theory of a shared etiologic factor. However, placenta previa and placental abruption do not share a common etiology in relation to a history of pregnancy induced hypertension, fetal growth retardation, and socio-economic factors.