The placenta acts as a barrier to many substances, but others freely or partially enter the fetal blood stream. 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 which causes maternal breast enlargement . Similar influences are commonly found in newborns, both boys and girls, by the third day after birth.
Breast hypertrophy is temporary, but 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 newborns system. The breasts should not be squeezed or massaged as this could result in an infection under the skin called an abscess . In addition to the breast enlargement, there may be some discharge from the nipples. This too is common and should be of no concern, disappearing within 2 weeks. The discharge is called witch's milk.