Monthly variation in preeclampsia prevalence: Washington State, 1987-2001

We investigated the extent of monthly variation in preeclampsia prevalence among primiparous women in Washington State who delivered from 1987 through 2001.

We identified all primiparae who gave birth to a singleton live infant during the study period using state birth records linked to hospital inpatient discharge data. We obtained a random sample and excluded women with pre-existing hypertension, renal disease, or diabetes mellitus or missing information on estimated date of conception (EDC) (n = 79 298). We defined preeclampsia (n = 6680) according to maternal birth hospitalization diagnosis codes and birth records. We calculated adjusted prevalence odds ratios (PR) and 95% confidence intervals (95% CI) of preeclampsia in each month of conception, relative to the month of lowest prevalence.

The prevalence of preeclampsia among primiparae was lowest among those conceiving in January (7.7%) and peaked among February, April, and July conceptions (8.9%). After adjustment, prevalence was significantly higher among women conceiving in February and April through August (PRs versus January: 1.14-1.19).

The prevalence of preeclampsia among primiparous women in Washington peaks among those conceiving in spring and summer. Exposures with similar monthly fluctuations may contribute to the etiology of preeclampsia.