Objectives: Aim of this project is determination of the correlation between the level of vitamin D in blood serum and duration of pregnancy in population in central Poland. Material and Methods: 25-Hydroxyvitamin D (25(OH)D) level was determined in blood serum, using enzyme-linked immunosorbent assay (ELISA). Standardized history of each patient was recorded. The history included: general medical history, data regarding the course of pregnancy and information about health-related behavior that could influence vitamin D concentration. Two hundred-and-one Caucasian women at childbirth were qualified into the study. The study group was divided into 2 parts: 100 patients who had a spontaneous premature birth and 101 patients who had birth at full term. Results: Vitamin D deficiency (< 30 ng/ml) was very common for both groups (69.6% of patients in the premature group and 72% – in the control group). Patients who had a premature birth had severe vitamin D deficiency (less than 10 ng/ml) more often than in the control group (34% vs. 14.2%, p = 0.001). Severe vitamin D deficiency increased the risk of premature birth but the association was not statistically significant in the multivariate regression model (odds ratio (OR) = 2.47, 95% confidence interval (CI): 0.86–7.15, p = 0.094). Conclusions: Severe vitamin D deficiency (< 10 ng/ml) may be the factor increasing the risk of preterm birth. Int J Occup Med Environ Health 2017;30(6):933–941