Between 2012 and 2016, a study was conducted in the United States on women who had changed their minds about having an abortion after taking mifepristone (RU-486), and before taking misoprostol, which is administered 24 to 48 hours later under FDA regulations.
Mifepristone is a progesterone antagonist; it blocks progesterone production. Administering progesterone therefore curbs mifepristone’s effects. Involving 547 pregnant women, the study showed that nearly half of the children were saved when progesterone was taken within 72 hours after mifepristone, regardless of how it was administered. It should be noted that in cases where mifepristone is only used, the embryo’s survival rate is 25%.
The study compared the various modes of administration, and two have proven to be particularly effective. In cases where intramuscular injection or high doses of oral progesterone were used, more than two in three children were saved.
When progesterone was taken in relation to gestational age was also a key factor. The later the stage of pregnancy, the more effective it was. That is only logical, as the effectiveness of mifepristone decreases over time.
On the other hand, the rate of congenital malformations remained the same as for conventional pregnancy. No evidence suggests that mifepristone is teratogenic. The study also showed no difference in the number of premature births or multiple pregnancies after taking progesterone.
The document concluded that the use of progesterone is “safe and effective”. Nevertheless, new studies would pinpoint the most effective dosage for even better results.
For further reading:
 Food and Drug Administration.