During the last 3 months of pregnancy, large amounts of calcium and phosphorus are transferred from the mother to the baby so that the baby's bones will grow.
A premature infant may not receive the proper amount of calcium and phosphorus needed to form strong bones. While in the womb, fetal activity increases during the last 3 months of pregnancy. This activity is thought to be important for bone development. Most very premature infants have limited physical activity, which may also contribute to weak bones.
Very premature babies lose much more phosphorus in their urine than do babies that are born full-term.
A lack of vitamin D may also lead to osteopenia in infants. Vitamin D helps with the body absorb calcium from the intestines and kidneys. If babies do not receive or make enough vitamin D, calcium and phosphorous will not be properly absorbed. A liver problem called cholestasis may also cause problems with vitamin D levels.
Diuretics or steroids can also cause low calcium levels.