The presence of endometrial tissue outside of the uterus. The endometrium is the mucus membrane inner lining of the uterus, with glandular cells and structural cells, both responding to oestrogen by increasing in size (the proliferative phase), the first responding to progesterone (the secretory phase); if there is endometrial tissue outside of the uterus, the tissue expands and shrinks in response to the estrus cycle, but the normal shedding of the menstrual phase can be difficult. The most common type of endometriosis is found in the fallopian tubes; the abnormal fallopian endometrial tissue can shed and drain into the uterus, but it hurts! It's funny, but little tiny ducts, like the ureters, bile ducts, and fallopian tubes really cramp. The colon and uterus are big muscular tubes and, when cramped up, cause rather strong pain. When one of those little bitty things gets tenesmus, your face gets white (or light tan), you start to sweat, shiver, and revert to a foetal position. Endometriosis that occurs around the ovaries or inside the belly and therefore can NEVER drain is a purely physical and medical condition, but fallopian presence of endometrium usually reaches its peak in the early thirties. It can be helped by ensuring a strong oestrogen and progesterone balance, thereby decreasing the tendency to form clots in the tubes, and to experience severe cramps every month