Parametrium is also called polymastia. The reason for this is that six to eight pairs of breast primordia occur along the anterior wall of the trunk from the axilla to the inguinal line (i.e., the breast line) at the sixth week of development. If the breast primordia outside the normal breast do not degenerate, they will develop into parametrium or polymastia. The incidence is 1% to 6%. Parametrium can occur anywhere on the breast line between the upper and lower extremities. The parametrium can occur anywhere on the mammary line between the upper and lower extremities, but clinically it is mostly seen in the axillary anterior axillary line and inguinal vulva. The shape of the parametrium varies, with the nipple, areola and mammary gland all together. There are also those who have only one of them. In a group of parametrial studies reported by Yunming Dai, the nipple and glandular type accounted for 19% and the glandular type accounted for 78%. The nipple-only type accounted for 3%. According to statistics, 29% of the parametrium had lactation function, but most of them showed swelling of the parametrium during pregnancy and lactation without milk overflow. However, it should be noted that the breast tissue contained in the parametrium has the potential to develop various breast diseases, including the development of parametrial cancer, so some scholars recommend prophylactic excision.