A recent study showed that 5% of pregnant white women and 29% of pregnant blacks in the northeastern United States had vitamin D deficiency, defined as a serum level of 25-hydroxyvitamin D [25(OH)D] less than 37.5 nmol/liter. About half of women in both groups had evidence of vitamin D insufficiency, with 25(OH)D levels of 37.5-80 nmol/liter. These findings are ascribed both to a lack of adequate solar exposure and a low oral intake of vitamin D even when a 400-IU supplement is taken. The racial disparity in preeclampsia suggests that vitamin D may be a relevant factor. This nested case-control study enrolled nulliparous pregnant women with singleton pregnancies who were followed from before 16 weeks' gestation to delivery in the years 1997-2001. Of 274 women in the study, 55 developed preeclampsia, defined as had new-onset hypertension and proteinuria developing after 20 weeks' gestation.
Women with preeclampsia were older than those without it and likelier to be non-Hispanic white, married, more educated, nonsmokers, and overweight when pregnancy began. Adjusted serum 25(OH)D levels in early pregnancy were 15% lower in women who later developed preeclampsia than in control subjects. Maternal serum levels of 25(OH)D below 37.5 nmol/liter in early pregnancy were associated with a 5-fold increase in the risk of developing preeclampsia, independently of race/ethnicity, season of the year, gestational age at sampling, prepregnancy body mass index, and educational level. Adjusting for calcium intake altered the findings only slightly. After adjusting for several possible confounding factors, a 50-nmol/liter decline in serum 25(OH)D more than doubled the risk of preeclampsia. The adjusted odds ratio was 2.4 with a 95% confidence interval of 1.1-5.4. The disparity in maternal vitamin D levels persisted at the time of delivery. Cord blood serum levels of 25(OH)D were significantly lower in newborn infants whose mothers were preeclamptic. These infants were twice as likely as those whose mothers were not preeclamptic to have a serum 25(OH)D level below 37.5 nmol/liter after adjusting for confounding factors. None of the associations was influenced by maternal race or ethnicity.
These findings suggest that maternal vitamin D insufficiency in early pregnancy is an independent risk factor for preeclampsia. If these results are confirmed, vitamin D supplementation in early pregnancy could be an effective and safe means of preventing preeclampsia.