Abstract
(1) Background: Pregnant women are at risk of vitamin D deficiency. Data on pregnancy outcomes in women with vitamin D deficiency during pregnancy are controversial, and prospective longitudinal data on vitamin D deficiency with consistent definitions in pregnant women are scarce. (2) Methods: The aim of this prospective longitudinal cohort study was to investigate 25-hydroxyvitamin D levels over the course of pregnancy and postpartum in singleton and twin pregnancies with regard to dietary and supplemental vitamin D intake and environmental factors influencing vitamin D levels, evaluated by a standardized food frequency questionnaire. (3) Results: We included 198 healthy singleton and 51 twin pregnancies for analysis. A total of 967 study visits were performed over a 3-year period. Overall, 59.5% of pregnant women were classified as vitamin D deficient in the first trimester, 54.8% in the second trimester, 58.5% in the third trimester, 66.9% at birth, and 60% 12 weeks postpartum, even though 66.4% of the study population reported daily pregnancy vitamin intake containing vitamin D. Dietary vitamin D intake did not affect vitamin D levels significantly. (4) Conclusions: The majority of pregnant women evaluated in this study were vitamin D deficient, despite administration of pregnancy vitamins containing vitamin D. Individualized vitamin D assessment during pregnancy should be considered to ensure adequate supplementation and prevention of hypovitaminosis D.