In detail, preimplantation genetic testing (PGT) implies medical testing that is conducted as a part of the in vitro fertilization (IVF) process, after the embryos have been created but before they have been transferred to the woman’s uterus. The procedure involves testing for embryos to identify whether there are no genetic abnormalities in them. In other words, this helps find specific genetic disorders, like cystic fibrosis or others, that may develop in an embryo. Genetic testing for monogenic disorders is an effective practice widely used in assisted reproductive technology to minimize the risks of a failed pregnancy or unhealthy fetus.
PGS, in turn, allows for a more narrow scope of examination — it helps identify an improper number of chromosomes in an embryo. In case such has been found, the embryo is left out, and only those with the correct number of chromosomes are transferred for implantation, hence reducing the chance of defects in the fetus. The methods used for screening are fluorescence in situ hybridization (FISH), which implies using fluorescent probes to detect DNA sequences, and array comparative genomic hybridization, used to detect the number of chromosomes and chromosomal abnormality.
For any family, facing issues with fertility is a challenging experience. Regardless of whether it is male or female infertility, the dream of starting a family then changes to a well-defined, step-by-step process of treatment to get pregnant. And when you finally succeed to conceive, you don’t want to risk the health of your future baby. This is when preimplantation genetic diagnosis (PGD) comes to the rescue.
While getting pregnant with reproductive medicine for IVF has its risks on its own, especially at an advanced maternal age (35+ years old), identifying the occurrence of any genetic disorder in the fetus will contribute to a healthy pregnancy and fetus. Therefore, PGD has to take place along with other testing intended parents have to undergo. In fact, the preimplantation genetic diagnosis can be performed on both fresh and frozen embryos, hence you can ensure the future baby’s health regardless of what type of material you use for fertilization.
With this being said, preimplantation genetic diagnosis (PGD) consists of preimplantation genetic testing for monogenic disorders and preimplantation genetic screening (PGS) to detect chromosomal abnormalities. In cases when particular diseases like hereditary cancer syndromes take place in any of the parents, testing is required to eliminate the chance of any threat to the life of the fetus. In particular, fertilization with preimplantation genetic examination consists of different types of preimplantation tests to ensure successful implantation and a healthy baby.
To understand the difference between these types of PGD, let’s put it simply: compared to preimplantation genetic testing, PGS does not help identify gene rearrangements and single-gene disorders.
THERE ARE 3 OPTIONS OF SCREENING YOU CAN CHOOSE FROM
- PGS-5 PGS-5 implies genetic screening of embryos by FISH on 5 chromosomes, namely 13, 18, 21, X and Y. This option is the least reliable among the 3 because of the limited chromosome set tested. However, it is still effective in detecting chromosomal abnormalities in embryos.
- PGS-9 Genetic screening PGS-9 is performed on 9 chromosomes, namely 13, 15, 16, 17, 18, 21, 22, X and Y. The method used for screening is FISH.
- PGS-24 Preimplantation genetic diagnosis (PGD) by FISH on 24 chromosomes is the most reliable screening option. In this case, every individual embryo will be screened for chromosomal abnormalities in 24 chromosomes. Due to the high number of chromosomes examined, the results of this testing type are the most accurate.
Prior to performing the procedure as a part of genetic testing for IVF, fresh embryos need to be created for the examination. Therefore, the patient needs to start the in vitro fertilization (IVF) process so that the egg is retrieved and fertilized to create an embryo. After the egg has been fertilized, it is now possible to start the screening and genetic testing of embryos. It’s also important to mention that frozen material can be used for the examination, too, in case no fresh eggs can be retrieved for fertilization.
The genetic selection for IVF starts with an embryo biopsy for embryo testing. This procedure implies removing cellular material from the embryos so that it can be further examined to get results for the diagnosis. There are different periods when the biopsy may take place, namely on days 0, 1, 3, 5 or 6 of their growth.
As it is the beginning of the embryo’s growth, there is still not enough material for the full and accurate examination. At this point, maternal DNA can be tested only since a polar body is the only material available for a biopsy.
On day 3, it's a cleavage stage, meaning that there is more material available for examination, which now allows testing the DNA of both parents. The material used for biopsy is one of the 8 cells that have grown in the embryo by this point. At this stage, PGS-5 and PGS-9 can take place, with the results being ready in several days. preimplantation genetic testing.
By this time, the embryos have grown to around 100 cells and developed into blastocysts, meaning that it’s the blastocyst stage, and the cells now consist of inner mass and trophectoderm. For the preimplantation genetic testing (PGT) and genetic screening (PGS), the inner cell mass is removed from the trophectoderm and can be safely used for the testing.
By examining a particular number of chromosomes, preimplantation genetic screening for IVF helps identify whether the embryo is euploid or aneuploid. If your PGS shows that the embryo is euploid, it means that the number of chromosomes it has is normal and can be transferred with no pregnancy risk.
Receiving a conclusion of your IVF preimplantation genetic screening with an aneuploid embryo implies that it carries a wrong number of chromosomes, such as additional or insufficient chromosomes. Aneuploid embryos will not be transferred since they increase the chance of pregnancy loss or abnormalities in the fetus.
Another result you can get from your screening is that an embryo is mosaic. In this case, the cells of an embryo contain both proper and abnormal numbers of chromosomes. These will still be transferred in case no euploid embryos are available, even though the chances for a healthy pregnancy are lower with mosaic embryos.
In such a way, you receive a preimplantation diagnosis that reveals whether the tested embryos are susceptible to any genetic diseases.
Risks with PGS
One of the risks is posed when there aren’t enough embryos for the biopsy or the DNA quality is too poor for the material to be useful for screening. These may translate into the risks of inaccurate screening results or their absence.
In cases where the screening is not possible due to insufficient material, such as because of poor DNA quality or damaged embryo cells, the embryos will still be transferred to the uterus since they have not been proven to be defective. However, the screening cannot be considered effective or helpful in this case.
Since preimplantation genetic testing is not 100% accurate because of these risks, a chorionic villus sampling is performed at weeks 11-13 of pregnancy to ensure that the fetus has no genetic defects.
Preimplantation Genetic Screening Cost
The cost of preimplantation genetic testing differs depending on several factors, such as the type of IVF genetic testing you choose, reproductive medicine you’ll take for treatment or the location of your clinic. Therefore, contact your doctor to find out your expected preimplantation genetic screening cost.
RELATED INFORMATION ABOUT EMBRYO GENETIC TESTING
To continue your research on how you can test an embryo for genetic diseases, visit our blog for more publications on embryo genetic testing, chromosome testing before IVF and in vitro fertilization with preimplantation genetic screening.
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