Although the exact cause of endometriosis is unknown, it is a complex disease with many different factors contributing to its development. A number of theories have been proposed to explain the origin of endometriosis such as;

• Retrograde menstruation: which is when menstrual blood containing endometrial cells flows back through the fallopian tubes and into the pelvic cavity at the time that blood is flowing out of the body through the cervix and vagina during periods leading to implantation and growth of menstrual cells outside the uterus

• Cellular metaplasia, which is when cells change from one form to another. Cells outside the uterus change into endometrial-like cells and start to grow.
• Stem cells giving rise to the disease, which then spreads through the body via blood and lymphatic vessels.

Other factors may also contribute to the growth of extrauterine endometrial tissue. These include altered or impaired immunity, localized complex hormonal influences, genetics and potentially, environmental contaminants.[i]

[i]https://www.who.int/news-room/fact-sheets/detail/endometriosis