In a normally cycling female, the corpus luteum produces 20 to 30 mg of progesterone daily during the luteal phase of the menstrual cycle. Progesterone is needed in hormone replacement therapy for menopausal women for many reasons, but one of its most important roles is to balance or oppose the effects of estrogen. Unopposed estrogen creates a strong risk for breast cancer and reproductive cancers. But progesterone levels may drop to near zero in some women. Because progesterone is the precursor to so many other steroid hormones, its use can greatly enhance overall hormone balance after menopause.
Progesterone also stimulates bone building and thus helps protect against osteoporosis.
Progesterone is preferable to the synthetic progestins such as Provera, because it is natural to the body and has no undesirable side effects when used as directed. If you have any doubts about how different progesterone is from the progestins, remember that the placenta produces 300-400 me of progesterone daily during the last few months of pregnancy, so we know that such levels are safe for the developing baby. But progestins, even at fractions of this dose, can cause birth defects. The progestins also cause many other side effects, including partial loss of vision, breast cancer in test dogs, an increased risk of strokes, fluid retention, migraine headaches, asthma, cardiac irregularities and depression. Estrogen is a potent and potentially dangerous hormone when not balanced by adequate progesterone.