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Blastocyst
Transfer
Embryos are traditionally
cultured until day three, or the 6-8 cell stage, before they are transferred
back to the uterus. Blastocysts are embryos that have been cultured to
day 5-6 and have divided into many cells and two distinct cell types.
There is an inner cell mass and an outer surface layer termed the trophectoderm.
"Weaker"
embryos do not survive to the blastocyst stage. Blastocysts are heartier,
have a higher implantation rate, and are more likely to survive. For these
reasons in IVF two blastocyst embryos can usually
be transferred to the uterus, which dramatically reduces  the
incidence of high order multiple births. Transferring 3 day three embryos
creates a higher risk of multiple births.
Blastocysts cannot
be used in all IVF cycles. The couple must have enough embryos to "risk"
extending culture for additional days since embryos can be lost during
culturing. A couple with two embryos on day two could ill afford to loose
any by culturing to day five. However, a couple with 6 embryos might be
a candidate for blastocyst transfer and cryopreservation. Dr. Muasher
and the embryologists assess each case on an individual basis.
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