Seasonal variation in the occurrence of pre-eclampsia

Objective To obtain evidence for seasonal variability in pre-eclampsia using the assumption that environmental factors may have a role in the causal mechanisms.

Design Cross sectional population-based study.

Population All 1,869,388 deliveries in Norway in the years 1967 to 1998.

Method For each month, the percentage of births complicated by pre-eclampsia was calculated. The relative risks of pre-eclampsia by month of delivery were estimated as odds ratios using the month with lowest risk as the reference category.

Results Mothers of children born in August had the lowest risk of pre-eclampsia, and the risk was highest in the winter months (for December adjusted OR: 1.26, 95% CI 1.20–1.31). This pattern was independent of parity, maternal age, year and place of living.

Conclusion The finding may provide a new clue for understanding the causes of pre-eclampsia. Environmental factors that show a similar seasonal variation should be investigated as possible causes.