Start supplementing before your baby is 2 months old. Begin by giving him at least 200 international units (IU) of vitamin D each day. Your pediatrician can help you determine the right time to begin supplementation; some nursing babies may need a month or two to adjust to breastfeeding before learning to accept a dropper. (A formula-fed baby only needs a supplement if he's not drinking 17 ounces a day by 2 months.)
Though 200 IU per day is the recommended amount, vitamin D is currently only available in multivitamin drops that contain 400 IU of vitamin D per milliliter. Rest assured, there is no harm in giving 400 IU per day.
Avoid direct sun exposure. Even though sunlight can boost your baby's vitamin D intake, it's important that you protect her skin from ultraviolet light. There's evidence now suggesting that the younger the age at which a child is first exposed to direct sunlight, the greater the risk of developing skin cancer later in life. The AAP recommends that infants younger than 6 months be kept out of direct sunlight and that older babies be protected with clothing and sunscreen.