Prenatal Determinants of Maternal 25(OH)D Levels at Delivery: The Role of Diet and Supplement Use in a Cross-Sectional Study in Greece.

Maternal vitamin D (25-hydroxyvitamin D, 25(OH)D) deficiency during pregnancy is associated with adverse outcomes for both mother and fetus. While vitamin D supplementation is commonly recommended, dietary and lifestyle factors influencing maternal 25(OH)D levels at term remain underexplored, particularly in Southern Europe. Aim: This study aimed to investigate prenatal determinants of maternal 25(OH)D levels at the time of delivery, focusing on dietary intake, supplement use, and seasonal variation. : We conducted a cross-sectional study on 248 pregnant women admitted for delivery at the General Hospital of Piraeus "Tzaneio" between September 2019 and January 2022. A structured questionnaire was used to assess prenatal intake of vitamin D-rich foods (such as fatty fish, eggs, dairy, and fortified products), supplement use (dose, frequency), sun exposure habits, and lifestyle factors. Maternal serum 25(OH)D concentrations were measured from blood samples collected at the time of admission for delivery. Statistical analysis included descriptive statistics and multivariate linear regression to identify independent dietary and supplemental predictors of maternal vitamin D status. : A high prevalence of maternal vitamin D deficiency (VDD) was observed, particularly during the autumn and winter months. Women who reported regular intake of vitamin D supplements (400-800 IU daily) had significantly higher 25(OH)D levels compared to those who did not. Dietary intake of vitamin D-rich foods was positively associated with maternal 25(OH)D status, although the effect size was smaller compared to supplementation. Seasonal variation, BMI, and limited sun exposure were also independent predictors. Both vitamin D supplementation and increased dietary intake were positively associated with maternal 25(OH)D concentrations at delivery. These findings underscore the importance of prenatal nutritional assessment and targeted supplementation strategies to prevent maternal VDD in Mediterranean populations.