For my last IVF, I was told the progesterone pessaries (3x 200mg per day) would only contribute about 10 to my P4 level. As per BDT's suggestion, there's a school of thought that the progesterone from pessaries or crinone goes straight to where it's needed (ie the uterus) rather than circulating in the bloodstream. That's why they're supposed to be more efficient than other options that have to enter the bloodstream (and go everywhere in the body, not just the uterus).