The hormone estrogen causes the womb lining or endometrium to thicken during the first part of the menstrual cycle, the American Congress of Obstetricians and Gynecologists. Estrogen is manufactured in the ovaries. If pregnancy does not occur during a cycle, then this lining is released from the body through monthly menstruation.
Progesterone is another hormone that is needed during this cycle. This hormone is released after ovulation. When there is no pregnancy during a cycle, the levels of estrogen and progesterone diminish to cause menstruation.
Many women can develop endometrial hyperplasia. This condition occurs when there is too much estrogen and a lack of progesterone, which happens if there is no ovulation. The excess estrogen causes an overgrowth of endometrial cells that can be a risk factor for cancer, states the American Cancer Society.