Seasonal variation in pregnancy hypertension is correlated with sunlight intensity

OBJECTIVE: To examine seasonality of pregnancy hypertension rates, and whether they related to sunlight levels around conception. STUDY DESIGN: Data were obtained for 424,732 singleton pregnancies conceived from 2001 through 2005 in Australia. We analyzed monthly rates of pregnancy hypertension and preeclampsia in relation to monthly solar radiation. RESULTS: Pregnancy hypertension rates, by month of conception, were lowest in autumn (7.3%) and highest in spring (8.9%). Higher sunlight intensity before delivery, but not around conception, was associated with decreased pregnancy hypertension (r = -0.67). Increased sunlight around conception may correlate with decreased rates of early-onset preeclampsia (r = -0.51; P = .09). CONCLUSION: The correlation between sunlight after conception and pregnancy hypertension was opposite to that hypothesized; however, sunlight levels before delivery did correlate with lower hypertension rates. For sunlight or ambient temperature to explain seasonal variation, the plausible exposure window is the period before delivery, but this may not apply to early-onset preeclampsia.