The cause of endometriosis is not fully understood. While there are several theories about the cause, none of them fully explain why endometriosis occurs. The most common theory is through implantation of retrograde menstruation.

Retrograde menstruation is when the endometrium flows backwards through the pelvis rather than through the vagina. This tissue then implants itself on organs throughout the pelvis and abdominal area and grows. While everyone with a period may experience some form of retrograde menstruation, the tissue does not implant onto organs in people without endometriosis.

Other theories around the cause of endometriosis include:

  • Genetic predisposition, where endometriosis may be passed down through families (hereditary) or linked to genetic variants.
  • Lymphatic or circulatory spread, where endometriosis travels through the lymphatic system. This theory may explain why endometriosis has been observed in uncommon locations (e.g., lungs, lymphatic vessels and lymph nodes, eyes, and brain).
  • Metaplasia, where one type of cell changes or morphs into a different type of cell. With endometriosis, metaplasia may explain how endometriosis cells appear spontaneously in the body.
  • Immune dysfunction, where the immune system is not able to fight off endometriosis. Since many people with endometriosis are more likely to have other conditions (e.g., migraines, hypothyroidism, depression, anxiety, and Lupus), it is believed that people with endometriosis may have a reduced immunity. However, it is not known if this causes endometriosis or is a result of endometriosis.
  • Anatomy, such as a tilted uterus or having a uterus and vagina that are shaped in a way that the menstrual flow is blocked.
  • Environmental factors, such as toxins in the water and lived environment, may disrupt hormonal regulation and contribute to endometriosis.

The development of endometriosis is not fully explained by any of these theories, but a combination of these theories is what scientists and physicians use to try to understand the cause of endometriosis and how it forms and spreads.

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