Blastocyst transfer not for everyone. (Less Success in ICSI Patients).

ORLANDO, FLA.--When blastocyst transfer debuted in the late 1990s, many experts predicted that it would be the wave of the future for infertility treatment.

But now many of those original enthusiasts have tempered their predictions with a few caveats, experts said at the annual meeting of the American Society for Reproductive Medicine.

Blastocyst transfer (BT) is the practice of culturing embryos in vitro for 2 days longer than in traditional in vitro fertilization (IVF) embryos and then transferring them to the uterus once they have reached the blastocyst stage, a full 5 days after fertilization.

Although BT was quickly adopted and remains on the list of services offered by almost all major fertility centers, most specialists now recognize that it is not an appropriate treatment for all patients.

"We don't do it in older patients or even younger patients if they only have a few embryos," Dr. Barry Behr, director of the IVF and Assisted Reproductive Technology laboratories at Stanford (Calif.) University, said.

In theory, BT results in the survival of the fittest embryos--those most likely to produce a pregnancy--helping embryologists select the best embryos to transfer back to the uterus. Because blastocysts are stronger and more developed than day 3 embryos, specialists are also more confident about transferring fewer of them, reducing the risk of creating multiple gestations.

But debate still continues as to whether culturing an embryo in a lab dish for 2 extra days might also harm a woman's chance of getting pregnant.