Effects on pregnancy outcome of changing partner between first two births: prospective population study.

OBJECTIVE To compare the effects on pregnancy outcomes of changing partner between the first two births with having the same partner for both births. DESIGN Prospective population study. SETTING Norway. PARTICIPANTS 31 683 women who changed partner between their first two births and 456 458 women with the same partner for both births. RESULTS After adjustment for maternal age and education, interval between births, and decade of birth, the risk of adverse pregnancy outcomes for the second birth was higher for women who changed partner between the first two births compared with those who had the same partner for both births: preterm birth (< 37 weeks; relative risk 2.0, 95% confidence interval 1.9 to 2.1), low birth weight (< 2500 g; 2.5, 2.3 to 2.6), and infant mortality (1.8, 1.6 to 2.1). For the first birth, the risk of these adverse pregnancy outcomes was only slightly higher for mothers who subsequently had a second birth with another partner. CONCLUSION Women who change partner between their first two births are at an increased risk of delivering a preterm, low birthweight baby with an increased risk of infant mortality compared with women who have the same partner for both births.