First Trimester Vitamin D Deficiency and Risk of Gestational Diabetes Mellitus in a Mexican Cohort

<b>Background/Objectives:</b> Vitamin D deficiency has been associated with an increased risk of adverse perinatal outcomes (APOs). This study aimed to examine whether vitamin D deficiency during the first trimester of pregnancy is linked to the development of gestational diabetes mellitus (GDM) in a Mexican population. <b>Methods:</b> A total of 404 pregnant women from the Biochemical and Epigenetic Origin of Overweight and Obesity (OBESO) cohort were included. Maternal vitamin D levels were measured between 11 and 14 weeks of gestation. Vitamin D deficiency was defined as a level below 20.0 ng/mL. The primary goal was to compare APOs between Group 1 (women with vitamin D deficiency) and Group 2 (women without vitamin D deficiency). Adjusted odds ratio (aOR) for APOs-including GDM, preeclampsia, preterm birth, miscarriage, cesarean section, and neonatal size-were calculated, adjusting for pregestational body mass index (BMI) and obesity, with 95% confidence interval (95% CI). <b>Results:</b> Vitamin D deficiency was present in 40.5% of women. Pre-pregnancy BMI and obesity were significantly higher in women with deficiency; other baseline characteristics did not differ between groups. Women with vitamin D deficiency had a higher risk of GDM (aOR 2.04, 95% CI 1.14-3.65, <i>p</i> = 0.01). No association was found between vitamin D deficiency and other APOs. <b>Conclusions:</b> The incidence of vitamin D deficiency in the first trimester was 40.5%. Early pregnancy vitamin D deficiency increases the risk of GDM among Mexican women. These findings highlight the importance of monitoring and supplementing vitamin D during pregnancy to reduce the risk of GDM.