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Genetic tests designed to weed out embryos that are unlikely to grow into healthy babies after in vitro fertilization (IVF) are often administered to couples receiving treatment even though it seems to have little impact on pregnancy rates. A new study involving higher-resolution genetic screening throws the practice into new doubt by showing that most of the cells in even healthy embryos have such chromosomal defects.

Evelyne Vanneste and her colleagues at the Catholic University of Leuven, in Belgium, used new, higher-resolution screening techniques to analyze cells from three- or four-day-old embryos from 23 fertile couples aged less than 35. Embryos are typically analyzed at this stage of development because less mature embryos contain less information, and more developed embryos are more difficult to transfer.

Vanneste and her colleagues found that more than 90 percent of the cells had some chromosomal abnormalities, a finding that goes some way toward explaining why humans have such low fertility rates in general. But it also means that some usable embryos may be discarded following screening.

Preimplantation genetic screening (PGS) usually involves either polymerase...