Association of seasonality with hypertension in pregnancy: a systematic review.

A systematic literature review was conducted to examine all academic, peer-reviewed studies of seasonal or monthly variation in the prevalence of gestational hypertension, preeclampsia, or eclampsia. The objective was to test the hypothesis that prevalence rates are highest during the winter months in non-tropical regions and during wet or humid periods in tropical climates. The authors searched the epidemiological literature indexed in PubMed, cross-referenced bibliographic materials, and reviewed personal archives. Of 60 abstracts and articles screened, 20 met the final inclusion criteria. Studies included were published between 1938 and 2010. Despite differences in setting, data sources, study design, outcome definitions, and control of known risk factors, 16 separate studies (11 non-tropical and 5 tropical) concurred that prevalence rates were higher for winter delivery in non-tropical regions or delivery during wet or humid periods in tropical climates. Although the reasons for these patterns are unknown, seasonal variation in infectious diseases, environmental triggers of asthma, vitamin D levels, physiological responses to cold temperatures, healthcare access, and nutritional intake may all play a role.