Season of Delivery and Risk of Hypertensive Disorders of Pregnancy in Los Angeles, California

Background: Environmental risk factors such as seasonal changes have been shown to be linked to hypertensive disorders of pregnancy. However, the effects of season of conception and delivery have been inconsistent and poorly studied in moderate climates such as Los Angeles. We aimed to study the effects of the season of delivery on the risk of hypertensive disorders of pregnancy. Results were verified via assessment of season of conception. Methods: We conducted a retrospective cohort study among 24,843 predominantly Hispanic women (2,360 clinically-confirmed cases of hypertensive disorders of pregnancy who delivered at the Los Angeles County + University of Southern California Women’s and Children’s Hospital between 1995 and 2008. The dry season was defined as April to October and the wet season as November to March. Logistic regression models were fit to estimate odds ratios for season of conception and delivery and risk of hypertensive disorders of pregnancy. Multivariable multinomial logistic regression models were then fitted to estimate odds ratios of season of conception/delivery and disease severity. 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). These mothers were also significantly more likely to be nulliparous (OR= 2.43, 95% CI: 2.21, 2.68), 25 years of age or older (OR=1.24, 95% CI: 1.12, 1.38), have diabetes (OR=1.62, 95% CI: 1.43, 1.85), have hypertension (OR=4.99, 95% CI: 4.14, 6.00), be diagnosed with infection (OR=2.14, 95% CI: 1.21, 3.81), or have premature deliveries (OR=2.11, 95% CI: 1.90, 2.33) when compared to controls. 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 examining the season of conception confirmed the results 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.