There is no clear agreement among doctors whether giving progesterone early in pregnancy will improve pregnancy outcome.
Many doctors feel that measuring progesterone levels in the blood is helpful when trying to find out if you ovulated or not. Progesterone levels are low if you do not ovulate, and after ovulation (midluteal, middle of the second half of the cycle) progesterone levels in a nonpregnant patient are generally at least 8-10 ng/ml. Much lower levels usually mean you did not ovulate though some doctor believe in something called 'luteal phase defect' which can prevent you from becoming pregnant..
A luteal phase defect does not prevent you from becoming pregnant but it makes the process less likely to happen and makes the likelihood of miscarriage higher. The addition of progesterone, as a catch up therapy, is unproven. There are physicians and programs that do not believe luteal phase defect is real: the strongest argument is a patient studies in Texas who had a biochemical inability to make "normal levels of progesterone" and who successfull carried her pregnancy without therapy.
There are differences in opinion as to what to do if progesterone levels are low after ovulation or early in pregnancy. Some doctors suggest giving progesterone (pills, shots, or suppositories) but many others do not treat unless you have IVF or injectable ovulation medications or if there has been proof of a preexisting low progesterone in nonpregnancy cycles and there are immune issues leading to miscarriages.
Many doctors feel that low or falling progesterone levels early in pregnancy are a sign of a failed pregnancy which cannot be stopped by giving progesterone supplements.
When you ovulate there is an area in the ovary called "corpus luteum" which is responsible for the productioon of progesterone. Progesterone levels rise after ovulation, and the rise can usually be detected about a week after ovulation. There are no "normal" progesterone levels after ovulation.