Risk Factors for Preeclampsia in Twin Pregnancies: A Population-Based Cohort Study

OBJECTIVE: To evaluate in twin pregnancies the characteristics parity, race, smoking, and age, known to be risk factors for preeclampsia in non-twin pregnancies. METHODS: All twin pregnancies (3407) and approximately twice as many singletons (8287) were assembled using Washington state birth certificates from the period 1984-1988. RESULTS: In singleton pregnancies, preeclampsia was more common in women who were younger, black, poor, nulliparous, and nonsmokers. In twin pregnancies, similar associations were found, but were only statistically significant for age, race, and parity. There were no significant differences in the risk factors between twin and singleton women. Logistic regression showed that twin pregnancy carries a relative risk (RR) of 3.5 (95% confidence interval CI 3.0-4.2), nulliparity an RR of 4.0 (95% CI 3.3-4.8), and black race an RR of 1.8 (95% CI 1.2-2.6) for preeclampsia. In each case, this risk is independent of the other risk factors. CONCLUSION: Twin pregnancy carries nearly a fourfold increased risk of preeclampsia, independent of race and parity, and the risk of a nulliparous twin pregnancy is 14 times that of a parous singleton pregnancy. Risk factors in a singleton pregnancy act similarly in a twin pregnancy. Thus, any pathophysiologic model for preeclampsia needs to account for the risk twin pregnancy poses as well as other risk factors, such as parity and race.