Racial/ethnic differences in the monthly variation of preeclampsia incidence.

Objective: The purpose of this study was to assess monthly patterns in preeclampsia risk separately by race/ethnicity.
Study Design: We conducted a cohort study of 20,794 white women and 18,916 black women who received care at 12 hospitals in the United States.
Results: Among white women, there was a significant U-shaped trend in the incidence of preeclampsia: the incidence was highest in winter months, reached its nadir in mid August, and subsequently increased through the fall months (P < .05). When compared with occurrences in January, the adjusted odds ratios were 0.65 (0.43, 0.99) for May, 0.76 (95% CI, 0.51, 1.13) for June, 0.64 (95% CI, 0.43, 0.97) for July, and 0.64 (95% CI, 0.42, 0.96) for August. Among black women, there was no association between month of delivery and preeclampsia risk (P = .81).
Conclusion: Our finding that the incidence of preeclampsia decreases in white women during the summer months and that no pattern exists in black women suggests that, in each subgroup, different or competing environmental exposures may be important for the pathogenesis of preeclampsia.