From Encyclopedia of Sex and Sexuality
One of the major female sex hormones, progesterone serves to prepare the uterus for the reception of a fertilized ovum. Progesterone is regularly produced in the ovaries by the corpus luteum. After ovulation, the endometrium is kept in a receptive state for the possible implantation of a fertilized ovum by increased levels of progesterone and estrogen. Production continues if the woman becomes pregnant. If not, the corpus luteum ceases to produce progesterone and estrogen, resulting in a rise in FSH (follicle stimulating hormone). The woman menstruates, and the cycle begins anew.
As a woman ages, the amount of progesterone she produces begins to decrease. A drop in progesterone levels is also believed to result in premenstrual syndrome, although no scientific evidence proves that this is the sole cause.