OBJECTIVE Pregnancy complications hypothesized to be related to vitamin D include preeclampsia, gestational diabetes, low birth weight, preterm delivery, Cesarean section and infectious disease. There have been a few studies which have demonstrated maternal serum vitamin D to be associated with preterm labor. The objective was to evaluate the serum vitamin D concentrations in Thai pregnant women with and without preterm labor and to find the prevalence of vitamin D deficiency and insufficiency in both groups. METHODS Blood samples were collected from pregnant women with and without preterm labor (matched for gestational age). Serum 25-hydroxyvitamin D (25-OHD) concentrations were measured by chemiluminescence method. RESULTS A total of 60 pregnant women were included into the study, 30 patients in the preterm labor group and another 30 patients in the control group. The serum 25-OHD concentration was 21.0±7.5 ng/mL and the prevalence of vitamin D deficiency was 48.3% in total group of pregnant women. The serum 25-OHD concentrations were not different between the preterm labor and the control groups (20.9±8.4 vs 21.2±6.7 ng/mL, p=0.91). The prevalence of vitamin D deficiency and insufficiency were not different between the preterm labor and the control groups (53.3% vs 43.3%, p=0.44 and 83.3% vs 90%, p=0.45, respectively). CONCLUSIONS The serum 25-hydroxyvitamin D concentrations and the prevalence of vitamin D deficiency and insufficiency were not different between the preterm labor and the control groups. The serum 25-OHD could not predict preterm labor in this Thai population.