Objectives To evaluate the relationship between vitamin D levels in women with gestational diabetes and the birth weight of their newborns. Methods This is a cross-sectional study nested in a controlled clinical trial performed in a public maternity hospital in Rio de Janeiro, with a sample composed of adult pregnant women diagnosed with gestational diabetes mellitus, without other chronic morbidities. The nutritional status of vitamin D was classified as 25(OH)D, being classified as: sufficient between 30 and 100 ng/mL; insufficient: between 20 and 29 ng/mL; and deficient below 20 ng/mL. In this analysis, the data regarding the vitamin D levels at the beginning of the study (T0) of the research were considered, and the dependent variable was the birth weight. Pearson's correlation was performed to evaluate the degree of correlation between two continuous variables, being considered as a level of statistical significance the value of P < 0.05. Results The mean maternal age was 35.4 (± 5.9) years (N = 11), and among these, 18.2% (n = 2) had adequate nutritional status, 27.3% (n = 3), overweight and 54, 5% (n = 6), obesity, according to pre-gestational BMI. Among the factors admitted as a risk for hypovitaminosis D (BMI > 25kg/m², brown/black skin color, marital status (married) and occupation), 45.5% (n = 5) presented 3 or more of them. As for nutritional status of vitamin D, 63.6% (n = 7) had insufficiency, 27.3% (n = 3), deficiency and 9.1% (n = 1), sufficiency. The initial mean concentration of 25(OH)D was 21.2 n/mL. It was observed that 72.7% (n = 8) of the pregnant women presented no gestational intercurrences at delivery or in the puerperium, while 18.4% (n = 2) presented unfavorable outcomes. The mean birth weight was 2842.5 (± 1251.1) grams and the mean gestational age at birth was 36.2 (± 6.5) weeks. With respect to neonatal outcomes was observed a significant positive correlation (r = 0, 900; P = 0.037) between serum levels of 25(OH)D at baseline and birthweight. Conclusions Preliminary data indicated that best status of vitamin D is related to higher birthweight values. Funding Sources FAPERJ (FUNDAÇÃO DE AMPARO À PESQUISA DO ESTADO DO RIO DE JANEIRO).