Objective: Vitamin D deficiency is common among pregnant women, and studies have shown that it is associated with obstetric complications such as gestational hypertension, gestational diabetes, and premature birth. Recent studies have shown that vitamin D deficiency in the second trimester of pregnancy is associated with premature birth. This study aimed to investigate whether vitamin D deficiency during pregnancy affects perinatal and long-term outcomes. Methods: In this prospective study performed between 2017 and 2021, we analyzed the data of 497 singleton pregnant women with no internal or surgical complications before pregnancy for obstetric, perinatal, and neurodevelopmental outcomes. Neurodevelopment was evaluated using Bayley-III tests, Gross Motor Function Measure, or chart review. Results: The maternal serum vitamin D level in the first trimester was 18.2±9.0 ng/mL and that in the second trimester was 20.5±9.0 ng/mL. In the first trimester, vitamin D deficiency (< 20 ng/mL) was found in 308 (62.0%). Among them, 92 women (18.5%) were in the very deficient group, in which the vitamin D level was less than 10 ng/mL in the first trimester. There were no differences in maternal age, body mass index, nulliparity, and previous preterm birth between the very deficient group and the normal group. Compared with the normal group, there were no differences in the rate of gestational hypertension, the rate of gestational diabetes, and the preterm birth rate. However, the rate of preterm birth before 37 weeks of gestation (adjusted odds ratios [aOR]=1.62, 95% CI[1.022.89], p=0.037) was significantly higher. In the very deficient group, the risk of developmental delay (aOR=1.67, 95% CI[1.013.38], p=0.041) was higher. Conclusion: This is the first study to analyze the effects of maternal vitamin D deficiency during pregnancy on long-term developmental outcomes as well as perinatal prognosis. Vitamin D deficiency in the first trimester may increase the risk of preterm birth and developmental delay in children.