Endometriosis is where cells that are similar to the ones in the lining of the womb (uterus) are found elsewhere in the body. Each month, these cells react in the same way as the ones in the womb - they build up, then break down and bleed.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/what-we-know-about-a-new-endometriosis-test-that-could-reduce-diagnosis-time-to-weeks