Progesterone is a steroid hormone synthesized from cholesterol and is important as an intermediate in the pathway to cortisol via pregnenolone, estrogens and testosterone. Progesterone induces the cyclic changes in the endometrium that allow implantation of the fertilized ovum. Progesterone is also responsible for maintenance of the uterus during pregnancy, suppression of uterine contractions until just prior to parturition and preparation of the breasts for lactation.

During the second half luteal phase progesterone levels increase sharply for a maximum of 5 – 10 days. If a women is not pregnant a steep decline in these levels is seen about days prior to the menstrual period.

In post menopausal it has been shown that a combination of progesterone plus estrogen provides an optimal hormonal profile for prevention of cardiovascular disease, osteoporosis and uterine protection. Progesterone has also been suggested to alleviate the symptoms of PMS.