Genesis Fertility Clinic Blog
searching: “preimplantation genetic diagnosis”
January 29, 2010
Preimplantation Genetic Screening (PGS)
There are so many factors that determine whether IVF will work, that is achieve a pregnancy. On the surface it might seem simple. After all, we are taking eggs and sperm, making embryos and putting the best looking (grade) embryos back into the uterus.
One reason is that many embryos that look normal are genetically abnormal. For example, we cannot tell if the embryo has an extra chromosome 21 which results in a child that would have Down Syndrome. Presently, we only examine the chromosomes of embryos if the couple is undergoing preimplantation genetic diagnosis for a known genetic disorder.
Why don’t we test every embryo to ensure we only transfer genetically normal embryos? Because it does not increase the chance that the woman will have a healthy child. This might seem surprising. After all, if you put only genetically “normal” embryos into a women, shouldn’t you have higher pregnancy rates because genetically abnormal embryos don’t implant as often? No, and the reason you don’t is that the process of testing an embryo’s genetic material can damage the embryo, thereby reducing the pregnancy rate.
Suppose a 33-year-old woman and her husband have 8 embryos. Currently, the best 1-2 embryos would be transferred into the woman, and she would have about a 65% pregnancy rate, about a 50% chance of a live birth, and a 0.002% chance of carrying a genetically abnormal fetus (because nearly all the genetically abnormal embryos will not implant or be lost at some point in the pregnancy). If we took those 8 embryos, tested their genetic maternal (called preimplantation genetic screening – PGS) and transferred only normal embryos, we would reduce the success rate by 25% (depending on the embryo biopsy technique) giving her a live birth rate of about 40%. Please note these numbers are for illustration only – different studies have shown different results. Some studies have shown a reduction in the success rate with PGS and some have shown no difference.
There have been just a few randomized trials examining the effect of PGS of embryos on pregnancy outcome and none have shown a benefit. In time, as the technique of embryo testing improves, I think PGS will be proven beneficial in improving pregnancy rates and the success of IVF, but for now it should not be performed routinely.
Dr. Beth Taylor, MD, FRCSC
Reproductive Endocrinology & Infertility