Background and aim. We planned this prospective cohort study in term newborn babies, with the objective to determine the incidence of vitamin D deficiency in infancy and to determine the level of vitamin D which triggers the physiological PTH axis of the body so as to differentiate truly deficient from sufficient vitamin D status. Material and methods. 96 participants at birth were enrolled and followed up till 9 months of age. Serum25OHD was estimated in cord blood at birth and at 14±1 weeks of life. 77 participants were followed up at 9 months for estimation of serum 25OHD, PTH, Alkaline phosphatase (ALP), calcium and phosphorus. Vitamin D deficiency was defined as serum 25OHD <15 ng/mL as per USIOM guidelines. Results. Serum 25OHD levels at 9 months of age (15.78±8.97 ng/mL) were significantly increased in comparison to the level of 3 months of age (14.04±7.10 ng/mL) and at birth (8.94±2.24 ng/mL). At birth all the participants (77) were deficient in 25OHD levels. It was found that 16/94 (17%) and 19/77 (24.7%) participants at 3 and 9 months of age respectively became vitamin D sufficient without any vitamin D supplementation. There was a significant inverse correlation between serum 25OHD and PTH concentration (r=–0.522; p<0.001), serum 25OHD and ALP (r=–0,501; p<0.001). It was found that reduction in serum vitamin D level to below 10.25 ng/mL results in surge of serum PTH. Conclusion. Vitamin D deficiency is common from birth to 9 months of age but incidence decreases spontaneously even without supplementation. Also large number of babies may be falsely labelled as vitamin D deficient with currently followed cutoffs. So a new cutoff for vitamin D deficiency needs to be established for neonates and infants.