Progesterone is a critical hormone both for initiating a pregnancy and for maintaining a pregnancy. The progesterone that is used to establish a pregnancy is made from the ovary while the progesterone that maintains a pregnancy is made by the placenta. The follicle is the unit in the ovary that makes progesterone. The follicle is a structure that is developed by the oocyte (egg).
In the first half of the menstrual cycle the oocyte is responsible for the development of a normal follicle. As the oocyte completes its development the follicle becomes larger as the fluid- filled space surrounding the oocyte enlarges. This is the structure that is seen on ultrasound. Also, as the oocyte develops, the follicle produces an increasing amount of estrogen. When the oocyte is developed, LH is released as a surge causing the follicle to rupture and release the oocyte. The LH surge also causes the follicle to switch from producing estrogen to producing progesterone.
Increasing levels of progesterone indicate that ovulation did occur and levels that reach normal value simply that the oocyte was normal. Low progesterone suggests that the oocyte was not structurally normal and this is an indirect indicator of oocyte quality. Normal progesterone levels in the second half of the menstrual cycle ( luteal phase) need to be determined 6-7 days after ovulation and at that time need to be > 8 ng/ mL.
Progesterone is frequently measured in the first trimester as an indirect indictor to see if the pregnancy is normal. If the progesterone level is < 12 ng/ mL then the miscarriage is as high a 70%. It is important understand that the progesterone is a result of a potential abnormal pregnancy and not the cause of the abnormal pregnancy. Many of the pregnancies that end in miscarriage are the result of a pregnancy with the wrong number of chromosomes and thus progesterone treatment will not save that pregnancy.