Parametrium is an enlargement of breast tissue outside of the normal breast and can occur in the axilla, anterior axilla, under the breast or in the groin. While animals have two rows of breasts during embryonic life, humans eventually have only one pair of breasts that develop normally, and the rest of the breast tissue gradually degenerates as it grows. In some people, the breast tissue does not completely degenerate and develops partially as it grows, known as the parametrium. The parametrium is part of the breast tissue and has no adverse effects on the body, but lactation can occur during menstruation, lactation or pregnancy.