PGD vs PGS
OVERVIEW
PGD (Pre genetic Diagnosis) and PGS (Pre Genetic Screening) are two different genetic tests, used by the doctors to investigate whether the In Vitro Fertilized embryo is suitable for implantation in the uterus and will result in a successful pregnancy or not.
Embryos with abnormal chromosomes or genes often result in either miscarriages, a failed treatment cycle or a child with a chromosomal or a genetic disorder.
PGD is performed to detect genetic defects or single gene mutations within the embryos, hence preventing certain genetic diseases to be passed on to the child.
Whereas PGS is performed to detect whether the embryo consists of any chromosomal abnormalities and if it has a normal number of chromosomes or not.
Pre Genetic Diagnosis
PGD is a technique performed prior to implantation of an in vitro fertilized embryo. It is performed on the embryo to detect any DNA change or mutation of a particular gene, called a single gene disorder or a Mendelian disorder. These disorders particularly pass from the parents to offspring and hence are important to be detected before implantation into the mother’s uterus is performed. The occurrence of single gene mutation is rare, however if both the parents are carriers of that particular genetic mutation then the chances of their offspring inheriting the disease are high. Therefore, PGD allows the parents to choose disease free and healthy embryos.
Steps involved in PGD technique:
- After the IVF process of egg retrieval and fertilization in a laboratory takes place, the embryo starts to divide into multiple cells.
- Around day 5 of cell multiplication, a few cells from the embryo are removed micro surgically for examination and the embryos are frozen.
- Then DNA evaluation is performed on the cells of each embryo to check for inheritance of a particular genetic mutation.
- When the PGD technique labels the embryos mutation-free, the embryo(s) are transferred in the uterus and couples are asked to wait for implantation and a positive pregnancy result.
- If there is an excess of embryos that are mutation-free, they are frozen safely to be used later for implantation, whereas embryos with mutation(s) are destroyed.
Who is a candidate for PGD?
- Carriers of single gene mutation disorders
- Carriers of sex linked genetic disorders
- Parent(s) with chromosomal disorders
- Women aged 35 years and above
- Women with more than one failed attempt of fertility treatment
- Women with recurrent pregnancy loss
Following diseases can be detected using PGD:
- Cystic fibrosis
- Sickle cell anemia
- Myotonic dystrophy
- Tay-sacs
- Thalassemia
- Fragile X syndrome
Pre Genetic Screening
PGS is a technique performed prior to implantation of an in vitro fertilized embryo. It is performed on the embryo to check for the chromosomal normalcy i.e. the normal number of chromosomes. A normal human contains 23 pairs of chromosomes that makes a total of 46 chromosomes. PGS evaluates the embryo for any addition of chromosomes or a missing chromosome, unlike PGD that is used to detect specific genetic disorders. Therefore, PGS allows the detection of genetic syndromes that are caused by chromosomal aberrations due to numerical alterations.
Steps involved in PGS technique:
- Initial steps are the same as in PGD, after cellular multiplication the cells from an embryo are removed micro surgically to be examined.
- The DNA from these cells is then evaluated to check for a possible chromosomal abnormality.
- Only the embryos that are disease-free are labelled as fit for transfer in the mother’s uterus.
- Additional embryo(s) are frozen to be used in the future, whereas the diseased embryos are destroyed.
Who is a candidate for PGS?
- Either parent with a family history of chromosomal abnormalities
- If the sperm is at risk of carrying a chromosomal disorder
- Women aged 35 years and above
- Women with more than one failed attempt of fertility treatment
- Women with recurrent pregnancy loss
Following diseases can be detected using PGS:
- Down syndrome
- Edward syndrome
- Patau syndrome
- Klinefelter syndrome
- Turner syndrome
SUMMARY
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PGD |
PGS |
| Checks for genetic disorders by evaluating an embryo for specific genetic mutation or a DNA change.
Can detect single gene mutations.
Performed to detect cystic fibrosis, myotonic dystrophy, sickle cell anemia, tay-sachs etc.
Can be used for gender selection.
Methods used in PGD; Fluorescent in situ hybridization(FISH), polymerase chain reaction(PCR) etc. |
Checks for genetic abnormalities by evaluating an embryo for chromosomal alterations numerically.
Does not detect single gene mutations.
Performed to detect Down syndrome, turner syndrome, klinefelter syndrome etc.
Can be used for gender selection.
Methods used in PGS; Array comparative genomic hybridization(aCGH), quantitative real time polymerase chain reaction (qPCR), next generation sequencing(NGS) etc.
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In vitro fertilization is a complex, time consuming and an expensive process, hence PGD and PGS are performed to increase the percentage of positive results of IVF. Both the tests hold immense importance and should be performed prior to implantation.
However PGD has a number of limitations and is considered an old technology where as PGS is more advanced and accurate. This is mainly because PGD test depends on single cell evaluation hence its effectiveness is questionable. In comparison PGS detects aneuploidy and mosaicism in embryos which allows scientists for the exclusion of such embryos from the Embryo Transfer and this can significantly improve success rates in IVF.
