Progesterone is sometimes called the "hormone of pregnancy" and it has many roles relating to the development of the fetus- Progesterone converts the endometrium to its secretory stage to prepare the uterus for implantation. At the same time progesterone affects the vaginal epithelium and cervical mucus, making the mucus thick and impermeable to sperm. If pregnancy does not occur, progesterone levels will decrease, leading, in the human, to menstruation. Normal menstrual bleeding is progesterone withdrawal bleeding. During implantation and gestation, it appears to decrease the maternal immune response to allow for the acceptance of the pregnancy. In addition progesterone inhibits lactation during pregnancy. The fall in progesterone levels following delivery is one of the triggers for milk production. The fetus metabolizes placental it in the production of adrenal mineralo- and glucosteroids.
Progesterone exerts its action primarily through the intracellular progesterone receptor although a distinct, membrane bound progesterone receptor has also been postulated. Progesterone has a number of physiological effects which are amplified in the presence of estrogen. Estrogen through estrogen receptors upregulates the expression of progesterone receptors. Progesterone, like pregnenolone and dehydroepiandrosterone, belongs to the group of neurosteroids that are found in high concentrations in certain areas in the brain and are synthesized there. Neurosteroids affect synaptic functioning, are neuroprotective, and affect myelination. They are investigated for their potential to improve memory and cognitive ability. Progesterone as neuroprotectant affects regulation of apoptotic genes.
Progesterone is naturally secreted by the ovary in the second two weeks of the menstrual cycle in reproductive age ovulating women. Progesterone or progesterone-like substances called progestogens or progestins are also ingested by women in birth control pills, menopausal hormone replacement therapy, or just sometimes to induce a menstrual period or regulate abnormal bleeding problems if menses are skipping or bleeding is irregular or prolonged. It has been used also as therapy for PMS syndrome and for women with infertility or frequent pregnancy loss. It has the identical chemical structure to the substance made in a woman's body by the ovarian corpus luteum (gland formed after an egg is ovulated each month). Actually the progesterone is now synthetically made but it behaves as best we know, just like the body's natural once it is absorbed into the blood stream. This