Seasonal variation in preeclampsia based on timing of conception

Studies have suggested that the incidence of preeclampsia may be partially dependent on the month or season of delivery. We sought to evaluate whether preeclampsia occurs seasonally in our population and whether the timing of conception or delivery is more strongly associated with risk.

Between January 1995 and August 2003, we identified 142 primiparous women with singleton pregnancies who met the American College of Obstetricians and Gynecologists' definition for preeclampsia and compared them with 7,762 primiparous control deliveries. We analyzed rates of preeclampsia by individual month and 3-month seasonal blocks based on conception and delivery. Data were analyzed with the chi2 test, and logistic regression and odds ratios were calculated where appropriate.

Preeclampsia occurred in 1.8% of singleton primiparous gestations (142/7,904). Cases were younger than controls (26.5 +/- 5.6 versus 28.0 +/- 0 6.0 years, P < .003), of similar race (97% white versus 96% white, P = .69), and equally likely to have a female child (45% versus 48%, P = .41). We found no significant association of month (logistic regression P = .20) of delivery with the risk of preeclampsia. There was a significant association of month (P = .003) of conception with risk of preeclampsia. Conception during the summer months had the highest risk (incidence 2.3%; odds ratio 1.7; 95% confidence limits 1.06, 2.75) compared with spring (incidence 1.4%). Fall (1.7%) and winter (1.6%) conceptions were associated with intermediate rates of preeclampsia.

We identified a seasonal variation in preeclampsia that appears to be more strongly related to timing of conception than to the timing of delivery. The highest incidence of preeclampsia was associated with conception in the summer months.

II-2.