Maternal Vitamin D Status and Preterm Delivery

Background: Identification of interventions for preterm delivery (PTD) prevention is a public health priority. Methods: We conducted a pooled nested case-control study from three prospective cohort studies to determine the association between maternal 25-hydroxyvitamin D (25OHD), a measure of vitamin D status, and singleton PTD. Participating cohorts are the Omega study (Washington), Pregnancy, Infection, and Nutrition Study (North Carolina) and Pregnancy Outcomes and Community Health study (Michigan). Maternal antenatal 25OHD levels were determined in banked blood specimens at a central laboratory using liquid chromatography-tandem mass spectrometry. PTD cases (n=870) were matched to term controls (n=1713) on cohort, maternal race (non-Hispanic black/white), gestational week at blood draw (median 19 weeks), and within 45 days. Serum 25OHD levels were categorized as <12 ng/ml (‘deficient'); 12-20 ng/ml (‘insufficient'); and >20 ng/ml (‘sufficient' and referent category). Spontaneous, and medically-indicated PTD were analyzed at <37 and <32 weeks. Conditional logistic regression analyses, which included adjustment for matching factors, were conducted. Results: Prevalence of 25OHD deficiency was 8.2% among all preterm cases and 6.5% among term controls. These levels varied by race: 26.2% among black cases and 22.9% among black controls; 2.0% among white cases and 1.1% among white controls. Significant odds ratios (OR) were observed for deficient 25OHD and medically-indicated PTD at <37 weeks: OR 2.3 (95% Confidence Interval 1.2, 4.2); and at <32 weeks: OR 6.6 (1.4, 31.3). Conclusions: Vitamin D deficiency is associated with increased risk of medically-indicated preterm birth. Improving maternal vitamin D status may reduce risk of preterm birth.