I don't know all the biology behind it, but basically in a 'natural' prg progesterone is produced by the ovaries after the release of the egg. During an IVF prg, so many eggs are released by the follicles that the progesterone levels are usually high anyway, but progesterone is used in the TWW (in the form of an injectoin or pessaries) to support the pregnancy.

Low progesterone levels have been linked to miscarriage but when I asked my FS if we should continue with the progesterone after my postive BT he told me that studies had shown that continued use of artificial progesterone had no affect on pregnancy outcomes - so high levels of progesterone alone aren't going to be enough to stop m/c from occurring.

I found this:
Progesterone functions to inhibit the smooth muscle in the uterus from contracting and decreases prostaglandin formation, both of which allow the fetus to grow with the expanding uterus.