4.2.3 Frequency and duration of follow up: Baseline and all follow up visits (3, 6, 9, and 12 months of life) will include anthropometric measurements and blood and urine analysis for (25(OH)D, PTH, calcium, phosphorus, creatinine) as well as bone markers and bone mass measurements. An additional blood sample will be obtained 1 month after starting the study to monitor blood calcium levels. Bottles of the supplement will be assessed for compliance at these visits. Other sources of vitamin D intake will be reviewed by diet history as well as determining the content of vitamin D in the breast milk. Maternal baseline information regarding demographics will be gathered at the initial visit and dietary vitamin D intake will be assessed at each visit.