Traditionally, it’s been believed that endometriosis is a disease where uterine tissue grows outside the uterus and attaches to nearby tissue and organs, creating painful lesions and inflammation.

However, according to a recent article on self.com, some medical professionals believe that the tissue isn’t actually uterine tissue but cells that are similar to uterine cells. These copycat cells are able to produce estrogen and should be shed the same way as uterine cells, i.e., during monthly menstruation.

MORE: Why increasing awareness of endometriosis is so important

While doctors may disagree on the type of cells causing the issue for women living with the condition, they do agree that it’s the cells not leaving the body each month that causes the problem.

Another issue dividing researchers is whether endometriosis is caused by retrograde menstruation — where instead of the blood and unwanted tissue leaving the body, it flows back into the fallopian tubes and into the pelvic area.

Some experts believe that many women suffer from retrograde menstruation (up to 90 percent), so why would only 10 percent of women have endometriosis? This school of thought believes women are born with the disease and will only begin showing symptoms once they begin their periods, as these women’s immune systems are not as well equipped to deal with clearing up debris from retrograde menstruation.

MORE: How endometriosis affects fertility

Endometriosis News is strictly a news and information website about the disease. It does not provide medical advice, diagnosis or treatment. This content is not intended to be a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or another qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this website.