The Interval Between Pregnancies and the Risk of Preeclampsia

Abstract

Preeclampsia is at least twice as likely in first as in second or later pregnancies. Apparently the risk declines in second pregnancies only if the partner is the same, suggesting that repeated maternal exposure and adaptation to foreign antigens lowers the risk of a repeat episode. It is possible, though, that the apparent increased risk with a new partner might simply reflect a longer interval since the previous delivery, a factor that may also make preeclampsia more likely to develop. This data analysis derives from a population-based Norwegian birth registry covering the years 1967 to 1998. The study group included 551,478 women having two or more singleton deliveries and 209,423 women having three or more. Preeclampsia complicated 3.9% of first pregnancies and, with the same partner, 1.7% of second and 1.8% of third pregnancies. Only 1.3% had a repeat episode when women with previous preeclampsia were excluded. Lacking such a history, the risk of preeclampsia in a second pregnancy increased steadily with the time since the first delivery (odds ratio, 1.16 per additional year). The risk had more than tripled 10 years after the first pregnancy. An increasing interval between second and third deliveries also correlated with an elevated risk of preeclampsia. The findings were unchanged after adjusting for advancing maternal age and the year of delivery. After adjusting for the interbirth interval, the risk of preeclampsia with a new partner was reduced (odds ratio, 0.73). An extended interval between pregnancies would seem to be a major risk factor for preeclampsia. Taking this interval into account, a change of partner between pregnancies is not a risk factor.