Genetic Testing of Embryos Leads to Births

With momentum building to transfer just one or two embryos, fertility clinics have begun to focus on choosing the one embryo that is most likely to succeed.

Traditionally, embryos have been selected for transfer based on a visual examination of their morphology, that is, their shape, the number of divisions, and other physical factors. But many embryos that look great under the microscope have undetected chromosomal abnormalities, such as missing or extra chromosomes.

The first IVF baby born in Britain using a new chromosome counting technique was reported this week, following on the footsteps of two babies born using the same screening tool, which is called microarray comparative genomic hybridisation (CGH), last October in Italy and Germany.

CGH is a genetic test that analyzes the chromosomes in an egg or embryo before the transfer stage of an IVF cycle. Healthy humans carry 46 chromosomes, 23 chromosomes contributed by the egg and 23 chromosomes contributed by the sperm. Eggs begin their development process with the full complement of 46 chromosomes, but half of them are shed into a small genetic bundle known as the polar body.

CGH can screen the chromosomes from up to five cells taken from an early embryo, called a blastocyst, or analyze the genetic quality of eggs by examining DNA in the polar body.

The test results take up to a week, so blastocysts are frozen and then thawed for implantation in a later IVF cycle. This allows the selection of a gestationally normal embryo. However, the polar body mirrors the chromosomes of the egg only, so CGH tests for chromosomal abnormalities derived from the egg, not the sperm.

The British researchers note that a randomized trial is necessary to be able to say exactly how much benefit CGH provides in improving IVF success rates, and further clinical studies are needed to reveal which patients will benefit the most.

At Weill Cornell Medical Center in New York, the IVF team led by Zev Rosenwaks, MD, uses preimplantation genetic diagnosis (PGD) to select healthy embryos in couples who may have missing or extra chromosomes or who may be at risk of having a child with a genetic disease.

For PGD, one or two cells is removed from the developing embryo for analysis. This allows the IVF team to identify embryos that do not carry the gene for certain inherited diseases, such as cystic fibrosis and sickle cell anemia. This capability greatly reduces the risk that these diseases will be passed on to children, and without lowering pregnancy rates.

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