Vitamin D status and gestational weight gain: A narrative review of maternal and fetal metabolic risk.

Gestational Weight Gain (GWG) affects maternal and fetal health. Vitamin D insufficiency, common during pregnancy, may impact weight control via metabolic, inflammatory, and endocrine pathways. However, the association between maternal vitamin D and GWG remain unclear. Therefore, this narrative review's objective is to integrate existing evidence on the correlations between maternal vitamin D and GWG, examine the effect of pre-pregnancy Body Mass Index (BMI), assess the cumulative effects of vitamin D deficiency and inadequate GWG on pregnancy outcomes, and assess the efficacy of vitamin D supplementation. A narrative review was conducted in accordance with SANRA 2.0 principles, with the search strategy reported in line with PRISMA-S guidance. A literature review was conducted in PubMed, Scopus, and Google Scholar to identify studies examining gestational weight gain (GWG) and serum 25-hydroxyvitamin D status published between January 2015 and November 2025. Observational and interventional studies were eligible for inclusion. After eliminating duplicates, reviewers independently evaluated 1,395 records; 14 studies were included. The prevalence of vitamin D deficiency and its bidirectional associations with maternal weight, gestational weight gain as a modifier of vitamin D's metabolic effects, combined effects on fetal and offspring outcomes, and supplementation trial evidence were narratively synthesized. Vitamin D insufficiency affected 30-85% of pregnant women worldwide. Bidirectional vitamin D-GWG relationships differed among studies, with prepregnancy BMI as a substantial effect modifier. In overweight women, low first trimester vitamin D levels were associated with 3.70 kg higher total GWG. Vitamin D's insulin resistance protection was greatly reduced by excessive GWG. High-dose supplementation (1600 IU/day) met biochemical needs but did not affect GWG. In one cohort, both low vitamin D and suboptimal GWG independently predicted small-for-gestational-age births. The association between maternal vitamin D status and gestational weight gain remains inconsistent across the current evidence base. Pre-pregnancy BMI may modify this relationship, with the strongest associations observed among overweight and obese women. Integrated strategies addressing both vitamin D adequacy and appropriate gestational weight gain, particularly among women with elevated pre-pregnancy BMI, should be explored in future research.