A clinicopathologic study of cutaneous endometriosis in 82 patients includes 28 lesions of the umbilicus, 42 of the lower abdominal wall, and 12 in the inguinal area, labia, and perineum. With the exception of five endometriomas of the inguinal area, every lesion occurred in a scar. Twenty-one arose without a preceding operation in the physiologic scar of the umbilicus, and 56 in surgical scars. The umbilical lesions were too precisely located to be logically explained by lymphatic spread, unless only the cells that find scar tissue are able to proliferate. Cesarean-section scars of 26 patients were involved, indicating a vastly greater incidence of endometriosis in cesarean scars than previously reported. This either casts doubt on the theory of transplantation as the usual pathogenetic mechanism for the lesions in surgical scars, or suggests that the endometrium of pregnancy is easier to transplant than is commonly believed.
Steck WD, Helwig EB. Cutaneous Endometriosis. JAMA. 1965;191(3):167-170. doi:10.1001/jama.1965.03080030011002