In women, progesterone is like a key.
The hormone helps turn on the processes of menstruation and ovulation. Progesterone also helps prepare women for pregnancies and maintain them as well. And since you cannot get pregnant without sex, progesterone also keeps sex drives high. About two weeks after a woman has begun her menstrual cycle, progesterone is released by the ovary. This is to set off the release of essential proteins for the lining of the uterus, to allow it to handle pregnancy. The proteins allow the endometrium - the uterus lining - to hold a fertilized egg.
When the fertilized egg implants, more progesterone is produced by the placenta. Progesterone levels in pregnant women can jump 10-fold from the level during her cycle, to about 250 mg. This additional progesterone is needed by the body to keep other eggs from being released and to spur breast-milk gland growth.
So you can see that progesterone is really important. Some say that progesterone creams are identical, chemically, to the naturally occurring hormone progesterone. So proponents talk about how everything the hormone progesterone can do, progesterone cream can do as well. The only difference? Progesterone cream has to be applied on the body and absorbed through the skin.