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What is the difference between PGS and PGD?
Pre-implantation genetic screening, or PGS, using aCGH (array Comparative Genomic Hybridization), is used to screen embryos for aneuploidy. Aneuploidy means an abnormal number of chromosomes. It can be either a trisomy (3 copies of a particular chromosome) or a monosomy (1 copy of a particular chromosome). Normal embryos should contain a total of 46 chromosomes: 23 from the egg and 23 from the sperm. The 46 chromosomes includes sex chromosomes: 1 contributed from the egg the X chromosome and 1 from the sperm containing either an X or a Y chromosome.
We strictly use trophectoderm (precursor of the placenta) biopsy on a blastocyst embryo for all aCGH cases since it allows us to remove more cells (about 5-7 cells) to get a more accurate result. It can be less harmful to the embryo. By performing aCGH on embryos, we can screen the embryos for aneuploidy to ensure only normal embryos are transferred back to the uterus. If an abnormality is found using PGS, the specific chromosome location of the abnormality is reported by the lab.
The below graph shows that with increase in maternal age and increased incidence of aneuploidy. Given this fact, it is extremely vital that patients consider performing aCGH to help increase success of a live birth.
Pre-implantation genetic diagnosis, or PGD, is used to screen embryos for specific single gene disordersthat both the female and male partner are known carriers of. If both female and male partner are known carriers of a genetic mutation, PGD can be performed on their embryos prior to transferring the embryo back to the uterus. Below is a list of the most common single gene disorders:
- Cystic Fibrosis
- Tay-Sacchs disease
- Fragile X
- Sickle Cell
- Spinal muscular atrophy (SMA)
PGD requires a special probe to be created specifically for the female and male patient’s genetic disorder to screen the embryos. The probe is used to detect if the embryo is affected by the genetic disorder, a carrier of the disorder or completely unaffected by the disorder.
Process of probe creation requires the female and male partner to either provide a blood sample or check swabs (depending on laboratory being used) in order to create the probe for the genetic disorder. The amount of time required to create the probe will be discussed with a genetic counselor from the outside laboratory creating the probe. Once probe is completed biopsy samples will be sent to the lab for testing.
At New Hope Fertility Center, we typically have patients that are both known carriers of a genetic disorder also complete PGS on the embryos as well to ensure the a carrier/unaffected embryo is transfer that is free of aneuploidy.
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