A common IVF test ignores some genetic abnormalities in the embryos

During in vitro fertilization (IVF), embryos undergo genetic testing before they are transferred into the uterus – but researchers have found that the widely used test cannot detect genetic abnormalities that form in embryos just before implantation. However, it is not clear what this means for selecting embryos that have the best chance of developing into a healthy pregnancy.

This procedure, called preimplantation genetic testing for aneuploidy (PGT-A), occurs about 5 to 6 days after fertilization. It involves removing a small number of cells from the outer layer of the fetus to check for extra or missing chromosomes, which may increase the risk of miscarriage. But this test is just a snapshot in time, as cells in the embryo continue to divide and multiply before implantation, which can lead to undetectable genetic changes.

So, Ahmed Abdel Baqi At the University of Cambridge and colleagues recorded the development of human embryos for 46 hours after they were thawed, to simulate the timeline between testing and implantation. It usually takes about one to five days for the embryo to implant after it is transferred to the uterus. Previous efforts to do this were only able to image embryos for 24 hours, because they are extremely sensitive to the light emitted by conventional microscopes. Instead, the team used an optical chip microscope, which illuminates only a thin slice of the embryo at a time, reducing light exposure and allowing for longer observation.

The researchers injected a fluorescent dye that binds to DNA into 13 human embryos, allowing them to monitor the formation of genetic abnormalities in real time. They observed 223 cells dividing across the samples, and found that 8% of the cells had a chromosomal abnormality. This occurs when chromosomes line up in the middle of the cell before it divides into two cells. Misalignment greatly increases the risk that the resulting cells will contain extra or missing chromosomes, which can hinder the transplant, increase the likelihood of miscarriage or cause conditions such as Down syndrome.

This suggests that “there may be a later time [genetic] “Changes in the fetus after the point at which we screen with PGT-A,” he says. Lily Zimmerman At Northwell Health in New York State.

These errors were limited to the outer layer of cells – which makes up the placenta – and not those in the center of the embryos, which develop into the fetus. Previous studies have shown that embryos with some genetic abnormalities in the outer cells can still lead to successful pregnancies. Therefore, it is possible that these genetic errors do not affect the ability of the embryos to survive, says Abdel Baqi.

“What this study shows to me is that there is still a lot of research needed in terms of screening embryos to see if [they] “It’s genetically normal or abnormal,” Zimmerman says. It’s not clear how genetic errors that occur between screening and implantation might affect an embryo’s viability, she says. The study also looked at only a small number of fetuses, so it’s difficult to know whether these findings apply to fetuses more broadly, she says.

Topics: