Progesterone is secreted by the empty egg follicle after ovulation has occurred, known as the corpus luteum. It is highest during the last phases of the menstrual cycle, after ovulation. Progesterone causes the endometrium to secrete special proteins to prepare it for the implantation of a fertilized egg. When fertilization does not occur, it prevents the body from creating and releasing more eggs in the later stages of the menstrual cycle.
If conception has occurred, progesterone becomes the major hormone supporting pregnancy, with many important functions. It is responsible for the growth and maintenance of the endometrium. It also suppresses further maturation of eggs by preventing release of LH and FSH (Follicle Stimulating Hormone). By relaxing the major muscle of the uterus, progesterone prevents early contractions and birth. It does however, also thicken the muscle, helping the body prepare for the hard work of labor. Finally, progesterone suppresses prolactin (the primary hormone of milk production), preventing lactation until birth.
Progesterone can be given as a supplement during early pregnancy if the doctor feels it is appropriate. Given as a pill, vaginal insert, vaginal cream or suppository, it can help boost the amount of progesterone your body produces, encouraging early pregnancy. It is very important to take this medication exactly as your doctor prescribes.
A form of progesterone is also given to induce bleeding in women who do not frequently have a period, which is common in women with PCOS. Extended periods of time without having a period can increase a woman's risk for endometrial cancer, one of the complications of PCOS. Therefore, it is very important to mention to your doctor if you have not had your period in a few months. The doctor may want to rule out pregnancy with a blood test before prescribing something to induce bleeding.