[Change of partner between births and adverse pregnancy outcomes]

Childbearing with different fathers has increased in western societies because of the higher frequency of divorce and remarriage. Pregnancy outcomes may differ between mothers who have changed partner between first and second birth and mothers who have the same father for both births. Women who change partner may have individual characteristics that increase risk of adverse outcomes, but change of partner could also be associated with unfavourable changes in behaviour.

The Medical Birth Registry of Norway was established in 1967 and includes information on approximately 1.8 million births until 1998. Both parents are recorded. We have compared pregnancy outcomes for 30 176 mothers who changed partner between first and second birth and 461 154 mothers who had the same partner for both births.

In relation to the first birth, the risk of adverse pregnancy outcomes was not clearly different between mothers who later had a second birth with another partner and mothers who had the same partner for both births. Offsprings of mothers who changed partner between first and second birth were at higher risk of infant mortality (relative risk 1.5, 95 % confidence interval, 1.3-1.7), preterm birth (< 37 weeks; relative risk 1.3, 95 % confidence interval, 1.2-1.4), and low birth weight (< 2500 gram; relative risk 1.5, 95 % confidence interval, 1.4-1.6), after adjustment for maternal age and level of education, interval between births, and time period.

Mothers who change partner between first and second birth are at increased risk of having a preterm, low birth weight baby with increased risk of infant death for the second child compared to mothers who have the same partner for both births.