Progesterone is a hormone of the progestogen group. Its functions include preparing the endometrium to be receptive and promoting embryo implantation. Progesterone is also responsible for maintaining the uterus in good condition throughout pregnancy.
This is why high levels of progesterone in the blood could indicate a possible pregnancy. However, this is not the only situation in which progesterone levels are high.
When there are ovarian cysts or a molar pregnancy, there are also high levels of progesterone in the blood.