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I found the study if you're curious on more details. It was basically done because there was a series of relatively poor evidence case series articles pushing the idea of mifepristone antagonization (the first pill in the two step process) with high dose progesterone to try and stop a medical abortion after the first pill but before the second pill. But case series are just basically cherry picked cases and very limited in what they can show. This practice was becoming more common though because of these, and so researchers wanted to learn more about what risks it would pose to women who did this, and if it was even effective in the first place (though this situation is very rare, less than 0.005% of women who take the first pill choose to try and continue their pregnancies).

https://pubmed.ncbi.nlm.nih.gov/31809439/

This is how participants were recruited:

We conducted this randomized, double-blind, placebo-controlled trial at the University of California, Davis Medical Center. We approached patients who had completed counseling and consent for a surgical abortion and were 63 days of gestation or less about study participation. Inclusion criteria were 18 years or older, English-speaking, singleton pregnancy, and willingness to delay the abortion by approximately 2 weeks. Exclusion criteria were medical contraindications to medical abortion per the mifepristone U.S. Food and Drug Administration label, an allergy to mifepristone or progesterone, or a peanut allergy (on-label contraindication to oral progesterone). The University of California, Davis, Institutional Review Board approved this study and all participants gave written study consent before beginning any study procedures.

Twelve people were enrolled, though two chose to stop the study early and were given surgical abortions before the planned two weeks. Three had to be transported by ambulance for bleeding, and further enrollment was halted.

What they found was:

Although the study sample size was powered to demonstrate a difference in continuing pregnancy rates between progesterone and placebo treatment after mifepristone ingestion, we could not evaluate this outcome owing to stopping enrollment for safety reasons.

Patients who use mifepristone for a medical abortion should be advised that not using misoprostol could result in severe hemorrhage, even with progesterone treatment. We stopped the study because of these complications and, thus, could not quantify the full extent of this risk. Because of the potential dangers for patients who opt not to use misoprostol after mifepristone ingestion, any mifepristone antagonization treatment must be considered experimental.

So basically, don't do this, and any "crisis pregnancy center" that advises this is putting pregnant individuals in clear danger. The American college of obstetricians and gynecologists knows what they're talking about when they say this should not be done.