Endometriosis is a medical condition that occurs when the lining of the uterus, called the endometrium, grows in other places, such as the fallopian tubes, ovaries or along the pelvis. When that lining breaks down, as the regular lining in the uterus does during menstruation, it has nowhere to go. This can cause cysts, heavy periods, severe cramps and even infertility.

About five million women in the United States have endometriosis, but the true number of cases may be much higher because not all women with the condition have symptoms, according to the National Institute of Child Health and Human Development (NICHD).

Endometriosis is classically defined as a gynecological disease, but this description may no longer reflect its true scope and manifestations, according to a 2021 review published in the Lancet. Since the full effects of endometriosis are not fully recognised and may extend far beyond the pelvis, it is now considered a systemic condition.

What are the causes of endometriosis?

The oldest and most known theory is retrograde menstruation implantation. "This would be the endometrial tissue flowing backwards through the fallopian tubes to the peritoneal cavity during menses, fixating there," Pastorelli Mosca told Live Science. "This tissue would theoretically suffer the same alterations of the in-uterus endometrium when exposed to estrogen, and grow."