Progesterone is a hormone that is solely formed in women. It is an extremely important hormone because performs its functions during pregnancy as well as in the monthly menstrual cycle. Progesterone causes increase in the cellular deposition in the endometrial lining of the uterus. It also causes the fallopian tube to secrete copious amounts of mucous in order to provide nutrition to the dividing ovum. And most importantly it causes the development of alveoli and lobules of the breasts. During pregnancy, progesterone relaxes the uterus and decreases the contractility. It also prepares the breasts for lactation. Women who undergo spontaneous abortions or miscarriages should use this hormone in order to bear children.