About Progesterone Progesterone works with estrogen to maintain reproductive organs, prepare for conception and pregnancy, and regulate menstrual cycles.  One of the many things estrogen does is to thicken and build the endometrium (the lining of a women's uterus) each month.  This is the body's natural way of getting ready for pregnancy.  Progesterone is one of the main components that causes this lining to shed each month when pregnancy doesn't happen.  This shedding is a woman's menstrual period. Before menopause, a few women may experience the absence of regular periods for six months or more.  This condition, known as secondary amenorrhea, is sometimes caused by not having enough progesterone. A woman's body makes one fourth or less the amount of progesterone after menopause.  When progesterone isn't there, the menstrual period becomes irregular or infrequent, and the lining of the uterus stays thick and full of blood.  Then, if that lining stays thickened month after month, it can be unhealthy.  This can increase the chances of developing hyperplasia, or an overgrowth of cells in the uterine lining, which can lead to endometrial cancer. If a woman's uterus is intact, her health care provider may presribe a progestogen with estrogen or an estrogen-androgen combination.  A progestogen: Helps shed the uterine lining and prevent development of hyperplasia May be prescribed for part or all of a woman's monthly cycle (the dosing schedule determines whether a period occurs) May lead to side effects such as spotting, menstrual cramps, bloating and drowsiness