A long interpregnancy interval is associated with preeclampsia. If some women experiencing a long interval between births had difficulty conceiving, subfecundity and preeclampsia may share a common etiology. Therefore, the authors examined the association between subfecundity and preeclampsia. By using interview data collected during the second trimester of pregnancy (1998-2001) from women participating in the Danish National Birth Cohort, they identified 20,034 and 24,698 singleton livebirths to primiparous and multiparous women, respectively, for whom preeclampsia information was available from hospital birth records. Among women with no known hypertension, the authors estimated a higher risk of preeclampsia in those with longer times to pregnancy (TTPs), after adjustment for maternal age, prepregnancy body mass index, and smoking. Compared with primiparas who became pregnant right away (referent category), the risk of preeclampsia increased with TTP and then stabilized for women taking 6 months or longer to conceive, whose risk of preeclampsia increased by 50%. Multiparas also had an increased risk, but only those reporting a TTP longer than 12 months (odds ratio = 2.47, 95% confidence interval: 1.30, 4.69). The authors found that a long TTP was associated with preeclampsia, supporting the hypothesis that some factors delaying clinically recognized conception may also be in a causal pathway for preeclampsia.