Since these growths are in fact pieces of uterine lining, they behave like uterine lining, responding to the hormonal cycle and trying to shed every month. The result is internal bleeding, degeneration of the blood and tissue shed from the growths, inflammation of the surrounding areas, and the formation of scar tissue. If the bladder is involved, there may be painful urination and blood in the urine. Irregular menstrual cycles and heavier flows are also associated with endometriosis, but usually women with severe endometriosis continue to have regular but painful periods. It's important to note that the amount of pain is not necessarily related to the extent or size of the growths.
Nobody knows the cause of endometriosis.
There are a few worthwhile theories, though. One is the theory of retrograde menstruation , also known as the transtubal migration theory . During menstruation, some of the menstrual tissue backs up through the fallopian tubes, implants in the abdomen and grows. Some researchers believe that all women experience some menstrual tissue backup, which is normally eliminated by their immune systems. A malfunctioning immune system or malfunctioning hormones allow this tissue to take root and grow into endometriosis.