I have a seven-month-old baby by IVF and ICSI. I had four embryos frozen for later use from my original stimulation cycle 16 months ago. I am considering using a couple of these instead of going through a stimulation cycle again. Is there an increased risk of abnormalities or complications when using frozen vs. fresh embryos?

ù Juli

The success rate with frozen embryo transfer (FET) varies widely from program to program and depends on many factors. The more cells present in the embryo and the healthier it appears, the more likely it will survive the freezing and thawing process. About 65-75 percent of healthy-appearing embryos will survive the freezing process.

Although it is called a frozen embryo transfer, we always thaw them and evaluate quality before transfer. Luckily, if the embryos are damaged by the freezing process, it is an all-or-nothing phenomenon -- either the embryo survives or it doesn't. I am not aware of any studies suggesting a higher rate of chromosomally abnormal offspring from transferring surviving frozen embryos as compared to fresh.

The implantation rate per embryo is somewhat lower with frozen embryos when compared to fresh, so we often transfer one more embryo than we might if transferring fresh embryos. We have noted an ongoing pregnancy rate of about 20 percent if three frozen embryos are transferred. Improved laboratory techniques should enable us to grow the embryos in the labs for a longer period before freezing. After four to five days, only the most fit embryos will appear healthy (viable) and will be frozen. As a result, fewer embryos will be frozen, but rates for survival and implantation will be better than with embryos frozen at earlier stages.