So realistically writing just doesn't make any sense then, why would sarah be a testing donor? Why would you test if unfertilized eggs can become fertile with science and use a fertilized egg to even test. There would be zero reason for Sarah to donate eggs because it would have zero affect on her.It's all explained in the story
The guy running the trial was obsessed with Sarah, his notes say that they were short on eggs, so they relied on a donor from the team. It turns out that only three pregnancies were derived from the trial, two of which miscarried, and the suggestion is that he replaced the trial patients' eggs with the donor eggs, that donor being Sarah.