Seasonal and Environmental Determinants of Maternal and Neonatal Vitamin D Status: A Cross-Sectional Observational Cohort Study in Urban Greece.

Background: Cutaneous synthesis of vitamin D depends primarily on exposure to solar ultraviolet B (UVB) radiation. Nevertheless, populations in the Mediterranean region, including pregnant women, continue to experience high rates of hypovitaminosis D. Pregnancy is a particularly vulnerable period due to increased physiological demands and reduced outdoor activity. The aim of this study was to examine the seasonal and environmental determinants of maternal and neonatal vitamin D status in an urban Greek population. Methods: We conducted a cross-sectional observational study on 248 pregnant women and their neonates admitted for delivery at Tzaneio General Hospital of Piraeus between September 2019 and January 2022. Serum 25-hydroxyvitamin D [25(OH)D] concentrations were measured and temporally matched with environmental variables including UV index, sunshine hours, ambient temperature, and PM2.5 levels. Results: Both maternal and neonatal 25(OH)D levels exhibited marked seasonal variation, with levels peaking in late summer and declining sharply in winter. A significant positive correlation was observed between UV index and vitamin D concentrations (r = 0.45, p < 0.001), while elevated PM2.5 concentrations were inversely associated with vitamin D status. Despite supplementation, insufficiency persisted in most neonates, particularly during the low-UV season. This underlines the need for comprehensive prenatal care strategies, integrating both supplementation policies and individualized nutritional counseling, to better secure maternal and neonatal vitamin D adequacy. Conclusions: Seasonal and environmental factors, particularly solar radiation and particulate air pollution, have a decisive role in determining maternal and neonatal vitamin D status, even in regions with abundant sunlight. These findings emphasize the importance of adaptive prenatal care strategies that combine supplementation with dietary counseling and take into account seasonal variation and air quality. In addition, the study provides novelty by integrating maternal-neonatal vitamin D status with environmental exposure metrics such as UV and PM2.5.