Objectives To determine the prevalence and risk factors of vitamin D deficiency in pregnant women and their infants at birth (cord blood) and at six months of age in Turkey, as well as to assess the compliance rates of families with vitamin D supplementation. Methods Serum 25-hydroxyvitamin D [25(OH)D] level was measured of the mothers before delivery and of the infants both at birth (cord blood) and at six months of age. Infants who received and did not take regular vitamin D supplements were compared in terms of 25(OH) levels. Independent risk factors were determined by multiple logistic regression analysis. Results The study included a total of 140 pregnant women and their infants. Vitamin D deficiency was found in 95.7% of the mothers. The prevalence of vitamin D deficiency was 87.1% in infants at birth but decreased to 5.8% at sixth month. 65.7% of infants received vitamin D supplements regularly. Despite regular vitamin D use, it was determined that 2.2% of the infants in the supplementation compliant group had vitamin D deficiency. Maternal age, maternal education level, and the number of siblings were determined to be determining factors on infants' 25(OH)D levels at six months (p < 0.05). Conclusions In Turkey, vitamin D deficiency still exists in both pregnant women and infants. Healthcare professionals and the public need to be more educated about the importance of regular supplementation. Serum 25(OH)D levels of infants should be tested periodically and personalized vitamin D supplementation planning is required based on test results.