Season of delivery and risk of hypertensive disorders of pregnancy in Los Angeles, California.

Background: Studies in temperate regions have observed an increased risk of preeclampsia in the winter, while studies conducted in tropical regions have found an increased risk during the rainy season. The effectof season of conception and delivery have been inconsistent and poorly studied in moderate climates such as Los Angeles.

Objectives: We aimed to study the effects of the season of delivery on the risk of hypertensive disorders of pregnancy.

Study design: We conducted a retrospective cohort study among 24,843 predominantly Hispanic womenwho delivered at a large, urban safety nethospital from1995 and 2008. Logistic regression and multivariable multinomial logistic regression were used to estimateodds ratios and 95% CIs.

Results: Mothers who delivered in the wet season had a significantly increased risk of developing hypertensive disorders of pregnancy compared to those who delivered in the dry season (OR = 1.11, 95 % CI: 1.02, 1.21). When stratifying based on disease severity, only mild preeclampsia indicated an association with season of delivery (OR = 1.12, 95 % CI: 1.01, 1.25). The effect for severe preeclampsia (OR = 1.12, 95 % Cl: 0.96, 1.31) and eclampsia/HELLP syndrome (OR = 0.80, 95 % CI: 0.49, 1.31) did not reach statistical significance, though power was limited in the more severe categories. Results evaluating season of conception as the outcome were similar to those for the season of delivery.

Conclusions: Among the predominantly Hispanic women who delivered at Los Angeles County + University of Southern California between 1995 and 2008, deliveries in the wet, winter season were associated with an increased risk of hypertensive disorders of pregnancy.