Is named because of its essential functions in maintaining conditions in the uterus that sustain pregnancy (pro-gestation).
During the menstrual cycle, estrogen causes the uterine lining to proliferate in preparation for implantation of a fertilized egg, and it stimulates a number of physiological events designed to prepare the body for pregnancy, such as stimulation of protein and lipid synthesis and mitotic activity.
Progesterone levels rise after ovulation and act to complete the preparation of the uterus for implantation, resulting in increased secretory activity and strengthening of the blood vessels. When progesterone becomes the dominant hormone in the secretory (luteal) phase of the menstrual cycle, it reduces the estrogen-binding capacity of the endometrial tissue. If implantation does not occur, progesterone levels fall and the endometrial lining is shed in the menstrual flow. If implantation does occur and pregnancy results, progesterone levels remain high to support the conditions required by the developing fetus.
The consistently high physiological progesterone levels seen in pregnancy would indicate that progesterone is safe as a therapeutic agent. Progesterone therapy is commonly used in women with infertility problems due to luteal progesterone insufficiency in order to raise circulating and endometrial progesterone levels to those of the normal luteal phase.
Progesterone has also been used to treat premenstrual syndrome (PMS), because the falling levels of progesterone prior to menstruation are thought to contribute to symptoms in some patients. However, there are few published studies.
With thanks to http://www.womeninbalance.org