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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.