Preimplantation Genetic Screening (PGS) vs. Preimplantation Genetic Diagnosis (PGD)
IVF is most likely to result in pregnancy and live birth when only genetically healthy embryos are transferred. Depending on a couple’s needs, two procedures may be used to ensure that this happens: preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD). Below, we will take a look at the difference between PGS vs. PGD.
Healthy vs. Unhealthy Embryos
In a normal embryo, there are 46 chromosomes in 23 pairs; each parent contributes half of the chromosomes. In a healthy embryo, there are two copies of chromosome 1, two of chromosome 2, and so on. The 23rd pair of chromosomes are the sex chromosomes – the “X” and “Y” chromosomes that determine whether the embryo will be male or female.
Even in couples who are healthy and presumed to be genetically normal, random chance means that a high percentage of embryos will not have the correct number of chromosomes. During natural conception, many embryos with chromosomal abnormalities are shed with the next menstrual cycle, often without ever implanting; others may miscarry later on. During IVF, transferring chromosomally abnormal embryos means that there is a higher chance of failure during the cycle. For couples who have a history of genetic illness, or signs of such issues, the chances of abnormal embryos are that much higher.
About Preimplantation Genetic Screening
Preimplantation genetic screening (PGS) is also known as comprehensive chromosomal screening (CCS). During this procedure, mature, healthy eggs are first fertilized by sperm. The embryos are allowed to mature for several days. Finally, an embryo biopsy is performed, which means that one or more cells from the embryo are removed and examined. The embryo biopsy is examined to ensure that there are 23 pairs of chromosomes in the cell. It is possible to perform PGS at several points in the embryonic development, before the embryos are transferred. Using this procedure, only embryos that appear healthy are transferred. When it comes to PGS vs.PGD, it should be noted that this procedure is recommended for couples who are presumed to be genetically normal.
About Preimplantation Genetic Diagnosis
Preimplantation genetic diagnosis (PGD) is a more precise procedure, allowing the embryologist to learn information about specific gene abnormalities in the embryo. During PGD, the eggs are first examined, and mature, normal-looking eggs are chosen to be fertilized. The following day, the embryologist examines the embryo, looking for features of a normal fertilized egg. At this point, a polar body biopsy can be performed. Two days later, the eggs reach the 8 cell (blastomere) stage, and another biopsy can be performed.
Several days from that point, the embryo has about 100 cells, and is known as a blastocyst. The blastocyst has two groups of cells, including the trophoblast, which develops into the placenta and embryo. Another biopsy can be taken at this point.
PGD can be used to screen for embryos with translocation, a structural abnormality that occurs between two chromosomes. It can also be used to actually diagnose embryos with certain genes or genetic mutations.
PGS vs. PGD
When it comes to PGS vs. PGD, it is important to understand that the procedures are beneficial for different populations. Any woman undergoing IVF is a potential candidate for PGS. It is recommended that couples consider PGD if they have one or more of the following issues:
- Recurrent, unexplained miscarriage
- Hereditary cancers
- Known carrier of genes for genetic disorders such as cystic fibrosis, hemophilia, or sickle cell anemia
If you are interested in IVF, Dr. Randy Morris would be happy to hear from you. To schedule your consultation today, please click below and enter your information or call IVF1 at (630) 357-6540.