PGS stands for pre-implantation genetic screening. It’s an innovative technique that is used to screen the number of chromosomes in embryos. This technology aids in the selection of genetically healthy embryos for transfer.
About 50% of human embryos derived via IVF (in vitro fertilization) are known to carry chromosomally
abnormalities. Even up to 40% of morphologically normal embryos harbor aneuploidies
PGS can improve pregnancy rates (70% pregnancy rate with PGS vs 40% without PGS) PGS reduces incidence of early miscarriages and thus the number of IVF cycles the patient must undergo.
If you choose to have PGS, you’ll firstly go through an IVF cycle to create embryos. The embryos are grown in the IVF lab until day 5. On the day 5 a few cells from the embryo are biopsied and tested for their chromosome content.
PGS treatment is recommended to patients who have experienced frequent miscarriages or repeated IVF failures. It is also suggested when either of the partners are of advanced age and in cases where a high incidence of sperm aneupolidy is diagnosed.
Embryo biopsy is a procedure where an embryologist removes a small number of cells from the outer layer of a 3 or 5-day-old embryo.
PGS testing can be performed on embryos biopsied at either day 3 or day 5. According to ESHRE guidelines, only a single cell is removed for a biopsy on day 3 as the embryo at this stage contains only 8 cells. However, a day 5 embryo has a few hundred cells, thus it is possible to extract up to 6 cells during embryo biopsy.
Though PGS is an invasive procedure, it has no adverse effect on the embryo. Embryo biopsy and micro-manipulation of embryos is being carried out in laboratories across the world and has been used for investigating the presence of genetic diseases for over 25 years. Studies so far have shown these procedures are perfectly safe, and do not cause any increased risk to children born as a result.
No genetic test can detect all potential genetic abnormalities. PGS testing is mainly performed to rule the possibility of embryos carrying aneuploid chromosomes, thus can only detect numerical changes in chromosome number. Specifically, PGS can identify any missing or extra chromosomes, PGS screening cannot rule out single gene disorders, balanced structural abnormalities, uniparental disomy, and genetic imbalances including deletions and duplications. PGS can detect some types of polyploidy but cannot detect polyploidy in which the sex chromosomes are found as a multiple of normal (triploidy 69, XXX and tetraploidy 92, XXXX or 92, XXYY).
The likelihood of a healthy pregnancy is increased to about 60% in women younger than 40 years when an embryo is identified as normal after PGS. As female aging has a strong association with an increased incidence of chromosomally abnormal oocytes, women over 40 years are reported to have a higher incidence of chromosomally abnormal embryos. When normal embryos are found, the likelihood of pregnancy is about 50%.
Although PGS is highly accurate, prenatal testing is advised due to the mosaicism in embryos. Mosaicism is a phenomenon in embryos where a segment of cells can carry aneuploid chromosomes while the remaining cells are normal. Prenatal testing is recommended to reconfirm the results of PGS. Moreover, prenatal testing may detect other abnormalities not tested for by PGS like microdeletions and duplications. Pregnant mothers conceived via IVF should discuss their options for prenatal testing with their Obstetrician. Noninvasive prenatal testing could be also an alternative.
There are many possible causes for miscarriage. About 50% of first trimester miscarriages are due to a chromosome abnormality. This may be related to a woman's age or a rearrangement in a parent's chromosomes that predisposes the couple to conceive pregnancies with chromosome abnormalities. PGS can be performed for either of these possibilities. For women over 35, the greatest risk may be for aneuploidy, a pregnancy with the wrong number of chromosomes, such as Down syndrome. PGS for aneuploidy screening offers these women a greater chance of a successful and healthy pregnancy.
#McCoy RC. Mosaicism in Preimplantation Human Embryos: When chromosomal abnormalities are the norm. Trends Genet. 33(7): 448–463 (2017).
#Harton GL, Munne S, Surrey M, et al. Diminished effect of maternal age on implantation after preimplantation genetic diagnosis with array comparative genomic hybridization. FertilSteril. 2013;100(6):1695–1703.
PGD is an abbreviated form of Preimplantation Genetic Diagnosis. PGD is a diagnostic test performed on embryos to detect the presence of single gene disorders such as sickle cell anemia, thalessemia, cystic fibrosis etc. This test aids in the selection of disease free embryos and helps the couple conceive a genetically healthy baby.
Since PGD is performed on embryo biopsy samples and multiple embryos are required for this purpose, it is necessary to undergo stimulating medications to ensure more than one mature egg can be retrieved, fertilized, and grown in the lab.
Infertility drugs are used to stimulate the release of multiple egg follicles which will facilitate the production of multiple embryos. IVF medications have been used for over 20 years. Studies investigating long-term effects of exposure to these medications have demonstrated that these medications do not put the patient at risk for ovarian or other cancers.
Human reproduction is very inefficient, and the likelihood of getting pregnant naturally in any given month is only around 10%. With the transfer of three embryos on average, the pregnancy rate of PGD is 30-35%. Many other factors may make the likelihood of getting pregnant such as quality of the embryos and age of the patients. The decision regarding how many embryos to transfer is made together with the patient to maximize the likelihood of achieving pregnancy, but also to minimize pregnancies with multiple fetuses.
Yes. It is necessary to identify the mutation causing the single gene disorder to proceed for a PGD case.
Aneuploidy is a numerical chromosome abnormality. It is recommended that PGS for aneuploidy screening
be performed in conjunction with PGD testing. Both tests can be done on the same embryo biopsy,
and therefore no additional risk to the embryo is implicated.
PGS for aneuploidy screens for abnormalities in chromosome numbers that could cause miscarriage or could result in the birth of a baby with birth defects and mental retardation. Down syndrome is an example of a genetic disease that is caused by a numeric chromosome abnormality.
After the probe has been developed, the IVF cycle and testing of embryos can be completed at the convenience of the patient and the clinic. The probe can be kept long term. Patients needing to delay IVF for longer than 6 months after the probe has been developed should make special arrangements with Anderson.
No, there is no pre PGD work-up time necessary for subsequent PGD cycles. This is because the same probe that was made for the couple can be used again for any future testing.
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