Healthy term babies undergo a physiological nadir in serum calcium levels by 24-48 hours of age. This nadir may be related to the delayed response of parathyroid and calcitonin hormones in a newborn. This nadir may drop to hypocalcemic levels in high-risk neonates including infants of diabetic mothers, preterm infants and infants with perinatal asphyxia. This early onset hypocalcemia which presents within 72 hours, requires treatment with calcium supplementation for at least 72 hours. In contrast late onset hypocalcemia usually presents after 7 days and requires long term therapy. Ionized calcium is crucial for many biochemical processes and total serum calcium is a poor substitute for the diagnosis of hypocalcemia.