NEW YORK (CBSNewYork) — A new advance could improve the chances of having a baby through in vitro fertilization.

It involves testing embryos and implanting those that are considered at least partially genetically abnormal.

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In 2015, Mary Jo and Shane Dunn lost their only child, Luke, to a rare cancer at 17 months old. They decided to try for another child through IVF. But after two years, six egg retrievals and $70,000, their hopes of a child came down to two remaining embryos that most clinics would not have implanted because, according to the Dunns, “scientifically they weren’t perfect.”

That led to normal twin girls, Riley and Kelsey.

The embryo testing the Dunns opted for is called PGS, preimplantation genetic screening. It takes a biopsy of a few cells from the embryo to determine if it is chromosomally healthy. It’s then classified as normal or abnormal.

Recent advances in this screening make it possible to detect when an embryo has a mix of normal and abnormal cells, called “mosaic” embryos. The twins grew from mosaic embryos.

Doctors believe, in some cases, those abnormal cells can self-correct or be expelled from the embryo. But it’s impossible to know if that will actually happen in any given embryo.

“Mosaic embryos can either fail to implant and you could miscarry, you could have a child with birth defects, or you could have a perfectly healthy baby,” says Mary Jo and Shane Dunn.

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Mosaic embryos do carry a higher risk of failure to implant and of miscarriage. Although according to one study, the higher the number of chromosomally abnormal cells in a mosaic embryo, the lower the birth rate.

Whether to transfer mosaic embryos using IVF is spurring debate among fertility experts.

“We have to learn which ones are more likely to make the baby and which ones are less likely,” said Dr. Jamie Grifo, director of the Fertility Center at NYU Langone Medical Center.

“We don’t have the full answer to the question of what is the probability that a mosaic embryo will be an affected baby or a healthy baby,” said Dr. Mandy Katz-Jaffe, scientific director at the Colorado Center for Reproductive Medicine.

For the Dunns, mosaic embryos were worth the risk.

Most PGS labs do not test for mosaic cells, they just report whether an embryo is normal or abnormal.

Ethicists also question whether infertile couples, desperate to have a baby, can fully appreciate the risk that a mosaic embryo could produce a baby with significant birth defects.

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A committee of the American society for reproductive medicine found earlier this year that they didn’t have enough information to form an opinion on what percentage of normal cells is needed for an embryo to be recommended for use.