The normal newborn breast diameter is about 1-2 cm. Breast swelling of various degrees can occur in about 70% of newborns, and is called by various names. Some of them include physiological breast enlargement, greater breast development, gynecomastia of the newborn, neonatal mastitis and breast enlargement.
The swelling may be soft or hard, and is recognizable by the third day of life. The breasts are somewhat tender, and occasionally, in about 5-20 percent of newborns, breast milk is also secreted from the swollen breasts for a couple of weeks. This used to be called witch’s milk under the impression that witches liked to drink it, but it is actually indistinguishable from ordinary human breast milk.
Pregnancy is associated with high levels of the female hormones estrogen. It passes through the placental circulation into the fetal bloodstream. This is responsible for many changes seen in the newborn as well including gynecomastia and vaginal swelling, just as it is in the pregnant woman. For this reason breast tissue in infants of both sexes responds to the elevated levels of the hormone by proliferating and enlarging. As delivery draws near, the level of estrogen falls and prolactin levels rise within the pituitary gland, in response. This leads to milk secretion.
It is not yet clear why only some newborns show greater breast development. It is thought that they may be hypersensitive to the effects of estrogen and/or progesterone. In at least one case, an overexpression of BRCA1 and BRCA2 receptors was found. This may be responsible, in which case more study is required on the future risk of malignant changes in such individuals.
Course and Prognosis
In most cases no treatment is required because the levels of estrogen and prolactin go down to normal over two weeks following birth, as they are metabolized in the newborn’s body.
In a few newborn infants, milk is not let out normally due to defects in canalization of the lactiferous ducts, or because of poor oxytocin secretion which promotes milk secretion. As a result, milk accumulates within the breasts, causing a galactocele to form. The entrance of infectious organisms into this receptive pool of milk may lead to the development of neonatal breast infection, which complicates about 25% of these cases. In about 70 percent this results in the formation of a breast abscess. This is more common in full term female infants between the third and fourth weeks after birth. In 9 out of 10 infants only one side is infected. Most cases are due to Staphylococcus infection.
Isolated gynecomastia subsides without any need for treatment. The breasts should not be squeezed as this only encourages the expression of milk by a hundredfold. Simple observation is all that is required.
However, when a breast abscess forms, it will require surgical treatment, either needle aspiration or surgical drainage of pus. As such, hospitalization will be necessary for surgery or intravenous antibiotic administration in 30-60 percent of these cases. Overall, about a quarter of neonates with breast enlargement go on to develop breast abscesses. Systemic spread is rare, but does occur.