What is a parametrium? The human mammary gland originates from the ectoderm. In the second month of embryonic development, six to eight pairs of mammary 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 breast primordia does not disappear, they will develop into extra breasts or nipples after birth, which is called an episiotomy (multiple breasts or nipples). Parametrium can occur anywhere on the breast line, but the most common site is at the anterior axillary margin. They can also be seen in the central axilla and abdominal wall. Occasionally, it can be seen in the groin, but rarely. There are three types of parametrium: (1) with breast tissue without nipples. (2) Nipple without nipple. (3) With nipple and mammary tissue. The parametrial breast with glandular tissue 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 size of the parametrial breast increases significantly during pregnancy, and those with nipples even secrete milk during breastfeeding. However, it may cause psychological anxiety and inconvenience to your life, and breast cancer can occur in about 0.1% of people. Because of the location of the secondary breast is different from the normal breast, sometimes it is easy to ignore. Therefore, when the parametrial breast is enlarged, the swelling is suspected to be a tumor.