The gradual thickening of the uterine lining also called Endometrial Hyperplasia is often caused by having excess estrogen without enough progesterone to balance it. During menstruation, the estrogen causes the lining to thicken in preparation for pregnancy. When the progesterone is made during the second half of the cycle and the progesterone levels drop, the lining is shed. And if the ovulation didn’t occur, progesterone is not made and the lining is not shed properly, the lining will keep on building gradually that may lead to cancer in some women.