Breast tissue begins to form in the fourth week of embryologic development along the ventral surface of the body as mammary ridges. The current prevailing theory of accessory breast tissue formation suggests that over time, these ridges regress everywhere except on the breasts, but in some individuals they may persist and contribute to accessory breast tissue (polymastia) and supernumerary nipples (polythelia). Occurrence is approximately 1% of the total population (females more commonly than males), although it may not be recognized until puberty. Rates have been reported as high as 5% in Japanese female individuals and as low as 0.6% in White persons.
Often patients are asymptomatic, and these lesions are noted during a routine exam, but patients may present with swelling or tenderness in the area that may worsen during puberty or pregnancy / postpartum.