Individual doctors may have clinical experience seeing women have recurrent miscarriages and then have successful pregnancies after progesterone supplementation, and so they might be strong believers in the idea that it actually does work.
Some doctors use progesterone supplements only in women who have already tested as having low progesterone. Doctors who use this approach may feel that the existing studies have not adequately screened patients, as many studies on progesterone currently have not differentiated women with low progesterone levels from women who have had miscarriages for other reasons. Again, they may have clinical experience seeing women with low progesterone levels end up carrying a baby to term after supplementation, so they believe that it works.
Given that the body naturally produces progesterone during pregnancy anyway, and many of the supplements on the market have a chemical makeup identical to the progesterone produced in the body, many doctors feel that supplementing women who have low levels is unlikely to do any harm even if it doesn't help. They decide to go with the philosophy that if progesterone can't hurt and might help, they may as well prescribe it.
Doctors who do not prescribe progesterone supplements may feel hesitant to prescribe any drug without clear indication that it works -- and they have a good historical ground to stand on. In the 1950s through the 1970s, doctors prescribed a drug called DES to pregnant women with the idea that it would prevent miscarriage -- the drug later turned out to cause numerous reproductive abnormalities in children.
Although most doctors feel progesterone supplements are probably safe, it's impossible to say whether in 10 or 20 years a study might uncover risks to using artificial progesterone supplementation during pregnancy. Doctors may wait for clear guidelines before proceeding.
Doctors may also feel that, until scientific evidence demonstrates a clear benefit, it is more likely that the low progesterone merely means an impending miscarriage, and that prescribing progesterone when a pregnancy is already destined to miscarry can delay the onset of the bleeding.
In discussing anecdotal evidence in which progesterone seems to work, doctors who reject progesterone supplements may point to the idea that women with recurrent miscarriages tend to have a high success rate even without treatment -- so in an uncontrolled setting, it is just as likely that a woman supplemented with progesterone would have had a successful pregnancy even without treatment.
Where It Stands
Doctors have different opinions on progesterone, so you will find some who prescribe it to every woman who has had recurrent miscarriages, others who prescribe it only to women who have low levels of progesterone, and still others who never use progesterone supplements (except for patients using artificial reproduction methods).
It is true that no one really knows whether or not progesterone supplements work, and that no amount of progesterone is going to make a difference in the ultimate outcome of women who have miscarriages due to chromosomal abnormalities or similar causes.
Progesterone supplements are probably safe. No one has found evidence that they can cause harm. If your doctor is advising you to use progesterone, be sure to discuss any concerns you may have and take both sides into account before making your decision. Similarly, if your doctor is not offering to prescribe progesterone, be sure to consider the reasons behind that stance before making a decision about your future care.
American Pregnancy Association, "Concerns Regarding Early Fetal Development." May 2007. 4 Mar 2008.
Nardo, Luciano G., and Hassan N. Sallam, "Progesterone supplementation to prevent recurrent miscarriage and to reduce implantation failure in assisted reproduction cycles." Reproductive BioMedicine Online. July 2006. Accessed 4 Mar 2008.
Oates-Whitehead, R.M., D.M. Haas, and J.A.K. Carrier, "Progestogen for preventing miscarriage." Cochrane Reviews. 11 Aug 2003. Accessed 4 Mar 2008.
Walch, K. and J. Huber, "Progesterone for recurrent miscarriage: truth and deceptions." Best Practice & Research Clinical Obstetrics & Gynaecology. 29 Oct 2007. Accessed 4 Mar 2008.