Seasonal variation in the hypertensive disorders of pregnancy in Denmark

Hypertensive disorders of pregnancy, including gestational hypertension and preeclampsia, are leading causes of morbidity and mortality for both mother and fetus. It has been proposed that vitamin D affects a number of biological processes involved in the pathogenesis of hypertensive disorders of pregnancy. Therefore, a seasonal variation in the risk of these disorders might be expected in areas such as Denmark with marked seasonal variation in sunlight exposure. In this study, we aimed to evaluate the existence of this association.

We used information from a cohort of 50 665 previously healthy, nulliparous women with singleton pregnancies. All women gave birth between 1989 and 2010 at Aarhus University Hospital, Denmark. Logistic regression analyses combined with the cubic spline method were used to estimate the seasonal variation for each outcome: gestational hypertension and preeclampsia, after adjusting for calendar time.

Of the 50 665 women included, 8.5% were diagnosed with a hypertensive disorder of pregnancy. The overall tendency was towards increasing risk when conceiving during spring and early summer, peaking midsummer, and subsequently decreasing steadily during late summer and fall to reach the nadir by winter. Seasonal variation was found for; gestational hypertension (P = .01); preeclampsia (P = .001) and early-onset preeclampsia (P = .014).

We found a seasonal variation in the risk of the hypertensive disorders of pregnancy in a large cohort of Danish nulliparous women. The highest risk was seen in women with the estimated date of conception in June and August, that is, during summer. Seasonal variation in vitamin D status may explain this association.