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The Mosaic Question

Next-generation sequencing has enhanced what pre-implantation genetic screening can pick up, leading to new uncertainties faced by physicians and parents, the New York Times reports.

It's uncovered that some 20 percent of embryos are mosaics of normal and abnormal cells, and that percentage increases with increasing maternal age, the Times adds. While it was long known that mosaic embryos existed, the Times notes that it's only been in the last year or so that they could be detected during an active in vitro fertilization cycle.

While mosaic embryos can develop into normal infants, the abnormal cells can also proliferate to lead to failed implantation, miscarriage, or an infant with serious defects. This uncertainty, the Times says, has lead many in the field to wonder whether mosaic embryos should be implanted as well as whether the PGS biopsy is representative of the inner cell mass from which embryos develop.

Columbia University Medical Center's Mark Sauer says he's not convinced that a small number of normal babies resulting from mosaic embryos "warrants a complete change in policy and standard of practice."

However, New York University's James Grifo is willing to transfer a mosaic embryo if the patient has no normal embryos and has undergone genetic counseling.

"A mosaic embryo does have potential for reproduction," he tells the Times, "but it could be anywhere on the spectrum from a healthy to a damaged baby, and we don't know where."