Preimplantation Genetic Screening
Daniel Kenigsberg, MD
August 2018 - PGS (preimplantation genetic screening aka PGD-A preimplantation genetic diagnosis for aneuploidy) involves removing cells from a blastocyst (day 5 or 6 of embryo culture) and determining whether that embryo has 46 chromosomes (euploid) or some other number (aneuploid: monosomy, trisomy, or polyploidy). At age 35, approximately 50% of good morphology blastocysts are normal by this chromosomal screening. Age is a crucial determinant because fewer than 20% of these embryos are chromosomally normal by age 40-42. Alternatively, approximately 70% of embryos from oocyte donors (usually < 32 years old) are chromosomally normal. This in large part explains the inefficiency of IVF.
Elimination of the variable of chromosomally abnormal embryos increases the chance of IVF pregnancy and significantly decreases the chance of pregnancy loss. It also allows for the ability to transfer 1 embryo at a time (eSET elective single embryo transfer) with good success and elimination of multiple gestations. Therefore, this technology has been rapidly adopted by IVF clinics over the past 5-10 years.
Probably the most important factor in recommending PGS is female age. As stated above, the random chance of a normal embryo is much higher in women <35 so the benefit of screening is less. Overall outcomes in women <35 with or without PGS are not nearly as dramatic as they are in women >37. Also important is the element of time to achieve birth as there is less of it in advanced age. Consider that in non-infertile women age 40-42, it may take 2 pregnancies to achieve 1 birth and pregnancy losses are time-consuming. All of this feeds into the approach that PGS should be offered and recommended to women having IVF beyond the age of 35.