Preimplantation genetic testing (PGT) is commonly performed in couples undergoing the IVF or in-vitro fertilization process for conceiving a child. PGT includes both preimplantation genetic screening (PGS) and preimplantation genetic diagnosis (PGD). Both these procedures are performed on the cells from the embryos in the embryology lab, but they both have different purposes and are used for the screening of different types of genetic problems.
PGS screens the biopsied cells from the embryo for any potential genetic abnormalities when there are no known disorders that can be potentially inherited. PGD, however, uses the same process to detect a specific type of disorder that has a high probability of getting passed down from the parents to the child.
Although not all couples undergoing fertility treatment need to have either of these tests done, PGS or PGD can help the reproductive endocrinologist in finding suitable healthy embryos to implant in the uterus (womb) of a woman during her IVF treatment.
In this article, we will be discussing who are the candidates for undergoing PGS and PGD and how these tests are done.
Why is it Important to Screen an Embryo Before Implantation?
The IVF procedure involves the fertilization of the fertilized eggs from a woman by the male sperm outside a woman’s body in a laboratory setting. The fertilized egg, now known as the embryo, is then transferred to a woman’s uterus. The successful implantation of the embryo into the uterine lining results in a pregnancy.
Screening this embryo before placing it into the woman’s embryo and implantation is important for the following reasons:
- About half of the preimplantation embryos that result from an IVF process are known to be chromosomally abnormal. PGT can help the doctor identify and transfer only the healthy embryos into the woman’s uterus.
- The success rate of pregnancy using PGS is found to be about 73%, whereas the success rate without using PGS is only 36%.
- It reduces the possible number of IVF cycles the patient needs to undergo before getting pregnant.
- Helps in checking any abnormalities in the chromosomal number (aneuploidies) and the selective implantation of only those embryos that have a normal number of chromosomes (euploid embryos).
Preimplantation Genetic Screening (PGS)
PGS test examines the chromosomal material present in an IVF embryo before implantation as the most common cause of the failure of embryo implantation and miscarriage are chromosomal abnormalities.
PGS test involves the removal of one or more cells from an IVF embryo to test for the chromosome number and check for aneuploidy. The test helps in improving the success rate of IVF by ensuring that only the euploid embryos having a higher chance of implantation to result in a live birth are transferred into the woman’s uterus.
Candidates For PGS
The ideal candidates for undergoing the PGS test are:
- Women above 35 years of age undergoing an IVF procedure
- Women with a history of recurrent miscarriages or pregnancy loss
- Women who had more than one failed attempt at IVF or other fertility treatment
- Either parent has a family history of chromosomal abnormalities
- When the male sperm is known to have a risk of carrying a chromosomal disorder
Diseases Detected Using PGS
The following conditions can be detected using PGS:
- Turner Syndrome
- Down Syndrome
- Klinefelter Syndrome
- Patau Syndrome
- Edwards Syndrome
Process of PGS
PGS test is done to check if the average chromosome count, that is 23 pairs or a total of 46 chromosomes, is present in an embryo. The process of PGS testing is done as follows:
- Once the process of female egg retrieval and fertilization in a laboratory for IVF is completed, the embryo developed in the laboratory starts dividing into multiple cells.
- Around the fifth day of cell multiplication, known as the blastocyst stage, a few embryo cells are removed micro-surgically for examination.
- The DNA from these cells will then be evaluated to check for any chromosomal abnormality.
- If the reports show an average chromosomal count, it suggests a euploid report. However, if the chromosome number is abnormal, it is considered to be an aneuploid report.
- In the case of an aneuploid report, embryo transfer is avoided, whereas, in the case of a euploid report, the embryo is transferred into the mother’s uterus.
Preimplantation Genetic Diagnosis (PGD)
PGD is a type of diagnostic procedure performed to test the material collected from an embryo for any presence of mutations that could be carried by one or both parents.
This test is usually carried out when one or both of the genetic parents have a known genetic abnormality. Every test prepared in PGD varies depending on the individual case. The test helps in the identification of the embryos having specific disease-causing mutations and the transfer of unaffected embryos.
Candidates For PGD
The possible candidates for PGD are:
- Carriers of single gene mutation disorders
- One or both parents having chromosomal disorders
- Carriers of sex-linked or X-linked genetic disorders
- Women who are above 35 years of age and undergoing IVF treatment
- Couples with a family history of single-gene disorder
- Couples who have had a child or children affected by a single gene disorder
- Women with a history of recurrent pregnancy loss
- Women who have had more than failed fertility treatment
Diseases Detected Using PGD
The following conditions can be detected using PGD:
- Sickle cell anemia
- Cystic fibrosis
- Tay-sachs
- Myotonic dystrophy
- Thalassemia
- Fragile X syndrome
Process of PGD
PGD is a method that helps in providing information on the genetic makeup of the cells present in a mature female egg. It can help in the detection of approximately 2000 single-gene disorders. The process of PGD is performed as follows:
- After the egg retrieval and fertilization process in a laboratory in IVF treatment, the embryo will start dividing into multiple cells.
- At around the fifth day of cell multiplication (blastocyst stage), about three to eight cells are removed from the embryo micro-surgically and the embryos are frozen.
- The cells are sent to the laboratory, where the DNA is evaluated on the cells of each embryo to check for the inheritance of any genetic mutation.
- If the PGD test reports suggest the embryo to be mutation-free, it is transferred to the woman’s uterus.
- If an excess number of embryos are found to be mutation-free, they will be frozen safely and can be used later for implantation, whereas the embryos having mutations will be destroyed.
Conclusion
IVF is an expensive and time-consuming procedure, and hence PGS And PGD are recommended to increase the possibility of positive results for IVF. Both tests are done before embryo transfer. While PGS is primarily used for the detection of chromosomal abnormalities in the embryo, PGD is used for checking any disease-causing mutations in the embryo.