Change in paternity, risk of placental abruption and confounding by birth interval: A population-based prospective cohort study in Norway, 1967-2009

Objectives: We examined abruption risk in relation to change in paternity, and evaluated if birth interval confounds this association. Setting: Population-based study of singleton births in Norway between 1967 and 2009. Participants: Women who had their first two (n=747 566) singleton births in the Norwegian Medical Birth Registry. The associations between partner change between pregnancies and birth interval were examined in relation to abruption in a series of logistic regression models. Primary outcome measures: Risk, as well as unadjusted and adjusted OR of placental abruption in relation to change in paternity and interval between births. Results: Among women without abruption in their first pregnancy, the risks of abruption in the second pregnancy were 4.7 and 6.5 per 1000 in women who had the same and different partners, respectively (OR=1.39, 95% CI 1.26 to 1.53). After adjustments for confounders including birth interval and smoking, partner change was not associated with abruption (OR=1.01, 95% CI 0.79 to 1.32). Among women with abruption in the first pregnancy, the association between partner change and abruption in the second pregnancy was 0.98 (95% CI 0.75 to 1.28). Interval