Seasonality of hypertensive disorders of pregnancy – A South Australian population study

Objectives: To investigate the seasonal variation of hypertensive disorders of pregnancy (HDP) in South Australia. Study design: Retrospective population study including all 107,846 liveborn singletons during 2007–2014 in South Australia. Seasonality in incidence of HDP in relation to estimated date of conception (eDoC) and date of birth (DoB) were examined using Fourier series analysis. Main outcome measures: Seasonality of HDP in relation to eDoC and DoB. Results: During 2007–2014, the incidence of HDP was 7.1% (n = 7,612). Seasonal modeling showed a strong relationship between HDP and eDoC (p <.001) and DoB (p <.001). Unadjusted and adjusted models (adjusted for maternal age, body mass index, ethnicity, parity, type of health care, smoking and gestational diabetes mellitus) demonstrated the presence of a peak incidence (7.8%, 7.9% respectively) occurring among pregnancies with eDoC in late Spring (November) and a trough (6.4% and 6.3% respectively) among pregnancies with eDoC in late Autumn (May). Both unadjusted and adjusted seasonal modelling showed a peak incidence of HDP for pregnancies with DoB in August (8.0%, 8.1% respectively) and a nadir among pregnancies with eDoB in February (6.2%). Conclusion: The highest incidence of HDP was associated with pregnancies with eDoC during late spring and summer and birth in winter, while the lowest incidence of HDP was associated with pregnancies with eDoC during late autumn and early winter and birth in summer. Nutrient intake, in particular vitamin D, sunlight exposure and physical activity may affect maternal, fetal and placental adaptation to pregnancy and are potential contributors to the seasonal variation of HDP.