Normal tissue from the endometrium (inside lining of the uterus) winds up outside the uterus. There are many reasons this might happen, nobody understands it exactly. Usually this endometrial tissue lands nearby in the pelvis, but it can occur in the abdomen & even the lung (very rare). Since the tissue is located where it doesn’t belong, it creates inflammation. And when hormones cause the normal menstrual period every month, this ectopic (out-of-place) tissue also reacts.
The main symptom of endometriosis is pain during periods, usually beginning a few days before. Since such ectopic tissue is usually in the pelvis, that’s where pain occurs. There may be chronic dull pain at other times if the tissue forms into a lump or mass. Pain during sex is also common, and infertility can be a frequent problem. Chest pain during the menstrual period, due to the very rare occurrence of endometriosis in the lung, can be very hard to figure out.
Physical examination is usually normal, as are laboratory tests. Imaging with ultrasound, and especially MRI, is a key to diagnosis. But the only way to be sure is by biopsy. Unfortunately, this requires some sort of surgery (usually laparoscopy), which entails certain risks as usual.
Basic treatment with birth control pills is often given without biopsy if a woman has typical symptoms, especially if an image is suggestive of the condition. But more complex treatment, which carries certain risks, usually requires diagnosis by biopsy (since the treatment is long-term).
One clinical problem is that the course of Endometriosis can be variable. About 30% to 45% of the time the condition gets worse, 30% to 40% stays the same, and 20% to 30% it gets better on its own. It almost always begins after puberty, and almost always ends with menopause.