Each month, these exiled endometrial implants respond to the monthly cycle, just as they would in the uterus-filling with blood, thickening, breaking down, and bleeding. The products of the process, however, cannot be shed through the vagina during menstruation. Instead, they develop into collections of blood that form cysts, spots, or patches. As the cycle continues these lesions may grow or reseed. The endometrial lesions vary widely in size, shape, and color.
Early implants are usually very small and look like clear pimples.
Over the years, they may diminish in size or disappear, but they may also grow, reaching sizes smaller than a pea or larger than a grapefruit. They can also form on the cervix, vagina, and even on the bladder and bowel. Very rarely, they have been reported in areas far from the pelvis, including the lungs and even the arms and thighs. A relatively uncommon form of endometriosis, called adenomyosis, occurs when implants invade the deep muscle layers of the uterus. In spite of the high prevalence of endometriosis in women all over the world, researchers have been unable to determine its cause.