While in the womb, a fetus is exposed to many of the chemicals present in the mother's bloodstream. The placenta acts as a barrier to many substances, but others freely or partially enter the fetal bloodstream.

Maternal hormones are among those chemicals that penetrate the placental barrier and have an influence on the fetus. During pregnancy the hormone estrogen is particularly high. This is the hormone that causes breast enlargement in the mother. Similar influences are commonly found in newborns, both boys and girls, by the third day after birth.

Effects:
Breast enlargement is temporary in an infant but is a common concern among new parents. This growth is short lived and should subside by the second week as the hormones are cleared from the newborn's system. The breasts should not be squeezed or massaged as this could result in an infection under the skin (abscess).

In addition to the enlargement, there may be some discharge from the nipples. This too is common and should be of no concern, disappearing within two weeks. The discharge is called witch's milk.

Newborn girls may initially have prominent labia as a result of the estrogen exposure. They sometimes also experience a type of vaginal discharge called pseudomenstruation due to the withdrawal of the maternal hormones. The discharge is white and occasionally tinged with blood. This condition is common and should not last beyond the first week of life.