The association between vitamin D deficiency in the first trimester and GDM development remains controversial in various ethnicities. We prospectively assessed whether pregnant women with vitamin D deficiency during early pregnancy had an increased likelihood of GDM development or poor fetal growth or pregnancy outcomes compared to those with sufficient vitamin D levels. Serum 25-OH-D measurements and fetal ultrasonograms were carried out at 12-14, 20-22, and 32-34 wk in 523 pregnant women. Each woman was screened for GDM at 24-28 wk. There were no differences in serum 25-OH-D levels at 12-14 wk or 22-24 wk of pregnancy between GDM and non-GDM women after adjusting for maternal age, BMI at prepregnancy, BMI at first visit, BMI at GDM screening, gestational age at sampling, previous history of GDM, vitamin D intake, and seasonal variation in sampling. The risk of GDM, insulin resistance, and impaired β-cell function had no association with serum 25-OH-D levels in crude or adjusted logistic regression analysis. GDM was not associated with maternal serum 25-OH-D deficiency during the first trimester or fetal growth during the first and second trimesters. Pregnancy outcomes such as miscarriage, Apgar 1, Apgar 5 and birth weight were independent of maternal serum 25-OH-D levels during the first, second and third trimester of pregnancy. In conclusion, neither GDM prevalence nor fetal growth during pregnancy is associated with vitamin D deficiency at the first trimester in Korean women. Pregnancy outcomes are also independent of maternal vitamin D status.