Progesterone is a steroid hormone, often referred to as a sex hormone. The word 'steroid' is a generic name for dozens of body regulators (hormones) made from cholesterol.

Cholesterol, the basic building block for the steroid hormone, gives them all a similar structure. Switch a few atoms around and the role of the hormone can change dramatically. Without sufficient cholesterol, we can't make sufficient steroid hormones. Some of the more familiar steroids are estrogen, progesterone, testosterone, the corticosteroids, and DHEA.

Progesterone is one of two main reproductive hormone groups, the other being the estrogens, made by the ovaries of menstruating women. It is primarily a hormone of fertility and pregnancy.

The three major functions of progesterone in our body are: - to promote the survival and development of the embryo and fetus - to provide a broad range of core biologic effects - to act as a precursor (building block) of other steroid hormones Progesterone is also made in smaller amounts by the adrenal glands in both sexes and by the testes in males. Progesterone is a precursor of other hormones such as estrone, estriol, estradiol, testosterone, and all the important adrenal cortical hormones.

The levels of progesterone in a woman's body rise and fall dramatically with her monthly cycles. At ovulation, the production of progesterone rapidly rises from 2-3mg per day to an average of 22mg per day, peaking as high as 30mg per day a week or so after ovulation. After ten or twelve days, if fertilization does not occur, ovarian production of progesterone falls significantly. It is this sudden decline in progesterone levels (as well as estrogen levels) that triggers a period/menstruation, and another menstrual cycle will begin.

When pregnancy occurs, progesterone production increases and the shedding of the lining of the uterus is prevented, preserving the developing embryo. As pregnancy progresses, progesterone production is taken over by the placenta and its secretion increases gradually to levels of 300-400mg per day during the third trimester.

If, however, a woman fails to ovulate during her cycle the result would be too little progesterone in her body, and estrogen dominates the hormonal environment. Progesterone is manufactured in the empty sack left behind by the released egg known as the corpus luteum. Unless ovulation takes place and the egg is released, progesterone will not be manufactured.

A fundamental key to hormone balance is the knowledge that when estrogen becomes the dominant hormone and progesterone is deficient, the estrogen can potentially become toxic to the body; thus progesterone has a balancing or mitigating effect on estrogen. Progesterone is the body's natural anti-estrogen.

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