The odds are too against a single embryo surviving thaw and making it to a successful implantation and pregnancy. I think at most clinics, the entire donation from a woman becomes yours, either for duration of the cycle, permanently, or until you in turn (if provisisions are made for this) donate them to another woman. I know some women would consider single embryos if combined with other small "lots." The embryos I chose, there were five blasts and I thawed all and transferred three based on their age, having been thawed and refrozen once, and not having been chosen for three prior (all successful) cycles. Two implanted and I have toddlers now. I preferred any twins I had to be genetically related to one another, so would not have chosen a singleton at all. Does that help a little?