Breast tissue is present in up to 90% of neonates resulting from the transfer of maternal oestrogen and progesterone, and gynaecomastia can occur in both girls and boys.
Enlargement of the tissue is a normal response to falling levels of maternal oestrogen at the end of pregnancy triggering the release of prolactin from the newborn's pituitary.
It is generally bilateral - although may not be symmetrical; if unilateral then consider a breast abscess or mastitis, especially if the infant is unwell and there is heat or erythema at the site.
The enlarged breast may discharge liquid, this usually resolves within a few weeks; squeezing this out increases the risk of complications including infection.