Congenital and acquired, acquired is generally due to acquired obesity or inappropriate underwear size, the use of strapless and semi-hood underwear in pursuit of fashion, which makes the chest unable to concentrate, and will cause local fat in the outer chest or armpit for a long time, thus forming auxiliary breasts. The reason for the formation of accessory breasts is that there are 6-8 pairs of mammary primordia on the two lines from armpit to groin during the embryonic period, and before birth, except for one pair on the chest, the others are degraded. If these mammary glands are not completely degenerated due to abnormal development, multiple breasts are formed, also known as polymastopathy. Accessory milk can be characterized by breast tissue, but no nipple; There are both breast tissue and nipples; No breast tissue, but nipples.