Endometriosis is characterized by the presence of tissue similar to the lining of the uterus (endometrial-like) located outside the uterus.  These fragments are different structurally, and behave differently, from the normal endometrium which is shed during a period (see Role of Estrogen Receptor-β and Progesterone Resistance in Endometriosis). This tissue causes pain, infertility, and organ dysfunction due to inflammation, invasion into structures in the body, and scar tissue. Endometriosis impacts approximately 1 in 10 individuals assigned female at birth and is rare in individuals assigned male at birth. Symptoms are often dismissed as “bad cramps” leading to an average 10 year delay in diagnosis.
Definition: “Endometriosis is defined as the presence of endometrial glands and stroma like lesions outside of the uterus.”
Parasar, P., Ozcan, P., & Terry, K. L. (2017). Endometriosis: epidemiology, diagnosis and clinical management. Current obstetrics and gynecology reports, 6(1), 34-41. https://link.springer.com/article/10.1007/s13669-017-0187-1