Because of the relative infrequency of neonatal breast hyperplasia proceeding to progressive mammary enlargement with secretion, it is of interest to present a case with a simple etiology without any apparent endocrinologic basis which might be readily overlooked.1,2 Hypertrophy of the breast is a rather common physical finding in the initial or discharge examination of the newborn of either sex. This usually is found to be a soft firm subareolar mass of breast tissue or only thick subcutaneous chest tissue. Infrequently, one also sees spontaneous discharge from the nipples.
Idiopathic precocious thelarche can be distinguished from neonatal breast hypertrophy by the presence in the former of the normal infantile chest contour prior to changes that make it appear more like that seen in an adult female. In neonatal breast hypertrophy the enlargement of the mammae does not regress.
Infant "lactation" has quite a long and interesting recorded history. Folklore
BLUESTEIN DD, WALL GH. Persistent Neonatal Breast Hypertrophy. Am J Dis Child. 1963;105(3):292-294. doi:10.1001/archpedi.1963.02080040294012