Progesterone helps prepare your body for conception and pregnancy and regulates the monthly menstrual cycle. Progesterone levels rise in the second half of the menstrual cycle.One of progesterone's most important functions is its role in thickening the lining of the uterus each month. The enriched endometrial lining is prepared to receive and nourish a fertilized egg.If a pregnancy occurs, progesterone is produced in the placenta and levels remain elevated throughout the pregnancy. The combination of high estrogen and progesterone levels suppress further ovulation during pregnancy. Progesterone also encourages the growth of milk-producing glands in the breast during pregnancy.If fertilization does not occur, estrogen and progesterone levels drop, the lining of the uterus breaks down and menstruation occurs.