How long after they are thawed are embryos transferred?
Blastocysts and embryos frozen at the cleaved stage can be transferred shortly after thawing (0-6 hours), but embryos frozen earlier on are frequently incubated for a day and transferred when they reach the cleaved or blastocyst stage. How long can embryos be frozen and still be viable? Theoretically, embryos may be frozen indefinitely, as no biological activity takes place during cryopreservation. In practice, it is unknown how long they can remain viable. The first frozen embryo transfers resulting in live birth took place in the mid-eighties. Since then, embryos frozen as long as 12 years have resulted in successful pregnancy. Freezing and thawing protocols continue to improve, making it possible that embryos frozen more recently may have better outcomes.
