Eclampsia is more prevalent during the winter season in Sweden.

Hypertensive pregnancy conditions have shown seasonal variations and have been related to low levels of vitamin D. In Sweden, the hours of sunlight per day are few in winter. We hypothesized that eclampsia would be more common in the winter season. We performed a population-based cross-sectional study of all singleton deliveries in Sweden during a 5-year period (n= 482,759 women). All parturients with eclampsia were included as cases and those without formed control group. The incidence of eclampsia was nearly doubled during winter season (odds ratio 1.9, 95% confidence interval 1.4-2.6) as compared to other seasons. The incidence of eclampsia during the period studied (1990-1994) was four in 10,000 pregnancies. We found that eclampsia was twice as common in winter and we introduce the alternative hypothesis that lack of sunlight exposure increases women's susceptibility to eclampsia, possibly due to vitamin D insufficiency.