Why Abnormal Embryos Can Result in Healthy Pregnancies
The Center for Human Reproduction’s Dr. David H. Barad explains that that the false positive rate of pre-implantation genetic screening (PGS) and embryos’ natural ability to correct chromosomal abnormalities means that many embryos that are being discarded after PGS as “abnormal” can grow into healthy, normal babies.
The existing evidence suggests that even when there’s evidence of aneuploidy (abnormal chromosome counts) in these embryos – if you go back and re-biopsy the embryo – as much as almost a quarter of the time – the second biopsy won’t agree with the first. So, there is variety in these embryos. We believe that some of that variety can lead to a healthy baby in spite of an abnormal diagnosis. On that basis, we’ve recently begun, special circumstances in people who have maybe gone through biopsies at other centers and told their embryos were abnormal but they were frozen; we’ve gone ahead and transferred embryos even when they’re abnormal. We’ve been doing this actually for some time. Using earlier techniques we transferred embryos when there appeared to be monosomies. And that’s because, if the monosomy were true, the embryo wouldn’t survive. So, you’re not worried about ending up with an abnormal baby, you’re just not going to have implantation. But if the monosomy is actually not representative of reality, then the baby has a chance of surviving. Especially using older techniques, in which it was harder to read monosomies, it seemed like a viable way to proceed. We’ve had three people who have gone on to have normal pregnancies after having such embryos transferred. So, we carrying that forward into the more modern techniques and beginning to ask questions about whether even with the new trophectoderm biopsy and quantitative PCR’s that people doing (which are very precise for the cells tested), might there still be some room in there based on what we call mosaicism (meaning the variety of chromosomal identities within the different cells) to still have a normal baby.