PGD / PGS Preimplantation Genetic Diagnosis (Embryo Screening)
Pre-implantation genetic diagnosis (PGD) - is a screening test used to determine if genetic or chromosomal disorders are present in embryos produced through in vitro fertilization (IVF). Preimplantation genetic diagnosis screens embryos before they are transferred to the uterus so couples can make informed decisions about their next steps in the IVF process. Embryos unaffected by the genetic or chromosomal disorder can be selected for transfer to the uterus.
Earlier PGD was done for couples who had serious genetic disorders to identify & discard the affected embryo and transfer good embryo to female uterus. With time scope of PGD has increased to include ploidy determination, PGS (Pre-implantation genetic screening) it does not look for specific disease but use PGS to identify embryo carrying de novo formed, not inherited, genetic errors.
Pre-implantation genetic screening (PGS) - is a comprehensive chromosome screening that examines all 24 chromosomes (22 autosomes plus X and Y). It may reveal
- numerical chromosomal abberations:
- extra copies of chromosomes (trisomies, tetrasomies)
- missing copies of chromosomes (monosomies, nullisomies)
PGS prevent recurrent miscarriages & does not get the future child affected by the most common aneuploidy disorders.
- Down syndrome (Trisomy 21)
- Edwards syndrome (Trisomy 18)
- Patau syndrome (trisomy 13)
- Turner syndrome (monosomy X)
- Klinefelter syndrome (gonosomal trisomy XXY)
These trisomies originate from meiotic errors that may arise during formation of gametes, especially in women.
In pre-implantation diagnosis it is very dangerous because it does not affect early embryo development but may lead to birth of a disabled child.
Thus PGS is suggested if meiotic errors are expected, Akruti has a full time genetic expert who can help the couple take an informed decision.
For diagnosis of diseases based on affected genes and chromosomal translocations as well as meiotic errors, both PGD and PGS require embryo biopsy, an invasive procedure for the removal of cell(s) needed for genetic analysis. The risk of mitotic errors – the third above mentioned source of genetic abnormalities in early embryo, can be significantly decreased with Time lapse system allowing detection of abnormal cleavages leading to chromosomal malsegregation resp. aneuploidy.
Embryo biopsy may be performed after 3 or 5 days of culture in the laboratory. The embryos are typically 8-cell embryos on Day-3 or blastocyst on day 5 and the process involves the removal of multiple cells, which are then sent to the genetic laboratory for diagnosis.
NGS (Next Generation Sequencing), in common with aCGH, is capable of analyzing all chromosomes. In contrast to aCGH, it has the advantage of higher sensitivity and ability to detect even aberrations that are found in an embryo in mosaic pattern. However, it can only be used on
Who Needs it?
- One or both partners have a history of heritable genetic disorders
- One or both partners is a carrier of a chromosomal abnormality
- The mother is of advanced maternal age
- The mother has a history of recurrent miscarriages
- For couples with heritable genetic disorders
- For couples with chromosomal disorders
Should I see a genetic counselor before undergoing pre-implantation genetic diagnosis?
Genetic counseling is an important step to determine if the preimplantation genetic diagnosis is an appropriate option for a patient. Akruti Fertility Centre has its own full-time genetic counselor of Reproductive Genetics. For couples undergoing IVF who are concerned that their child may inherit a genetic disorder or chromosomal abnormality, a genetic counselor is available to discuss options and can advise patients.