Parametrium, or multiple breasts, is a common abnormality of breast development in women. It is often an underdeveloped tissue, mostly resembling a baby’s breast, or only a little skin pigmentation is seen, and there may be a little skin thickening in the center, resembling a tiny nipple. Some have only mammary glands and some have only nipples, but there are also complete breast bodies (nipples, areolas, and glands) that are visible in the axillae and are larger. The parametrium is also painful and swollen before menstruation, and increases significantly during pregnancy, and even produces milk during breastfeeding. Parametrium is an abnormal development in which the mammary glands have disappeared completely, and it can occur in any part of the breast line. The most common site is at the anterior axillary margin, but it can also be found in the central axilla and abdominal wall. Occasionally, it can be seen in the groin, but it is rare. There are three types of parametrium: (1) with breast tissue without nipple. (2) There is nipple without breast tissue. (3) There are nipples and breast tissue. The parametrial breast is affected by the same sex hormones as the normal breast, with cyclical changes, premenstrual swelling and pain, and the same common diseases as the normal breast, such as hyperplasia, cysts, and parametrial adenocarcinoma. The human mammary gland originates from the ectoderm. During the second month of embryonic development, six to eight pairs of mammary gland primordia are formed on the ventral surface of the embryo, from the axilla to the inguinal “breast line”. Normally, by the 9th week of embryonic development, all of them have degenerated and disappeared, except for the pair of mammary primordia in the anterior thoracic region between the 5th ribs, which are preserved and continue to develop. If one (or more) of the remaining mammary gland primordia does not disappear, they will develop into extra breasts or nipples after birth, which is known as an epigynum (multiple breasts or nipples). It is very common for women to have parametrium, and this is related to the choice of underwear. Due to the wrong perception in the past, women prefer to choose a smaller size in their underwear in the hope of achieving the effect of squeezing out the cleavage. Because of the small bra, the steel ring and cups are not enough to cover the breasts, and the long time squeeze, the breast fat around the breasts will be squeezed out, and the mammary glands will develop towards the armpits which are not covered, and the side breasts will come out. The above content introduces what exactly is the problem of side breast. Parametrium generally has no effect on the human body, but it may give you psychological uneasiness and inconvenience in life, and about 0.1% of people can develop breast cancer. Because of the location of the parametrial breast is different from the normal breast, sometimes it is easy to ignore, so when the parametrial breast increases, the inconvenience of living with a swollen fast suspected to be a tumor, go to the doctor.