Risk for hypertensive disorders during pregnancy varies by season
Women who conceive in the spring and summer may have increased risk for developing preeclampsia and gestational hypertension, according to research published in Acta Obstetricia et Gynecologica Scandinavica.
Researchers attributed the association to seasonal variation in vitamin D levels.
“Our results are of great interest, as vitamin D may have caused the observed seasonal variation in the hypertensive disorders,” Christine Rohr Thomsen, MD, of the Aarhus University Hospital, Denmark, said in a press release. “It has long been assumed that vitamin D affects the pathogenesis of hypertensive disorders of pregnancy — including preeclampsia — and our results support this hypothesis.”
A previous study also showed that birth season could impact health. These findings revealed that women born in the spring or summer had increased risk for CVD mortality compared with those born in the winter or fall. Researchers identified seasonal fluctuations in nutrition availability, infections, temperature, air pollution and sunlight exposure as potential factors driving the association.
In Denmark, where the current study was conducted, reduced vitamin D synthesis during months with limited sunlight exposure — February and March — cause up to 90% of all Danish women to become vitamin D-deficient, according to Thomsen and colleagues. In months with the most sunlight exposure — August and September — vitamin D deficiency is observed in less than 20% of women.
Thomsen and colleagues used data from the ongoing Aarhus Birth Cohort at the Aarhus University Hospital to estimate the seasonal variation of gestational hypertension and preeclampsia. The study included nulliparous women with single births between 1989 and 2010, excluding those with prepregnancy primary hypertension, CVD, renal diseases, metabolic disorders, diabetes, gestational diabetes and lupus.
Among 50,655 women included in the analyses, researchers found that 8.5% experienced a hypertensive disorder of pregnancy — 3.9% had preeclampsia and 4.7% had gestational hypertension.
Thomsen and colleagues identified seasonal differences in the risk for gestational hypertension (P = .01), preeclampsia (P = .001), early-onset preeclampsia (P = .014) and late-onset preeclampsia (P = .01).
The risk for gestational hypertension and preeclampsia were highest in women who conceived during the summer and lowest in those who conceived in the fall or winter, according to the researchers. For early-onset preeclampsia, the risk was highest in the spring and lowest in the fall.
Women with an estimated conception date in June had the highest risk for preeclampsia (OR = 1.17; 95% CI, 0.94-1.45), and those who conceived in August had the highest risk for gestational hypertension (OR = 1.35; 95% CI, 1.11-1.64). Women who conceived in May had the highest risk for early-onset preeclampsia (OR = 1.81; 95% CI, 1.02-3.22).
The associations did not change when researchers adjusted for pre-pregnancy BMI, maternal age, ethnicity, smoking habits or in vitro fertilization.
“This may support the existence of an association between vitamin D and the hypertensive disorders of pregnancy although more direct measurements of the vitamin D status at different times during pregnancy should be studied in relation to pregnancy complications in order to draw more certain conclusions,” Thomsen and colleagues wrote. – by Erin Michael
Disclosures: The authors report no relevant financial disclosures.