The association between vitamin D levels in the second trimester of pregnancy and gestational diabetes mellitus

25-hydroxyvitamin D (25(OH)D) affects glucose metabolism by increasing insulin secretion and insulin receptor expression. However, whether 25(OH)D deficiency will increase the risk of gestational diabetes mellitus (GDM) has not been clearly reported. The purpose of this study is to assess the relationship between vitamin D levels in the second trimester of pregnancy and the risk of GDM.According to the inclusion and exclusion criteria, 247 pregnant women came to the fourth hospital of Shijiazhuang (The affiliated obstetrics and gynecology hospital of Hebei Medical University) for obstetrics were investigated during the period of January 1, 2019 to December 31, 2020. The levels of 25(OH)D in the second trimester (16-20 weeks) and oral 75 g glucose tolerance test (OGTT) at 24-28 weeks of pregnancy were reviewed. The sociodemographic data were collected from questionnaire. Multivariate logistic regression was used to analyze the relationship between vitamin D levels and GDM.The incidence of GDM in the observation group (25(OH)D ≤ 26 ng/ml) was higher than that in the control group (25(OH)D > 26 ng/ml) (p = 0.039). Compared with control group, the observation group had significantly higher level of fasting plasma glucose (FPG) (4.7 [4.5-5.0] mmol/L vs. 4.6 [4.4-4.8] mmol/L, p = 0.012). In the whole study, the level of 25(OH)D was negatively correlated with FPG (r = - 0.164，p = 0.010). After adjusting for age, pre-pregnancy BMI, parity and adverse pregnancy history, compared with the observation group (25 (OH) D ≤ 26 ng/ml), the risk of developing GDM decreased by 50.9% in control group (25(OH)D > 26 ng/ml) (odds ratio [OR] = 0.491, 95% confidence interval [CI] = 0.243-0.989, p = 0.047).Adequate vitamin D levels during the second trimester of pregnancy may reduce the risk of GDM.