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, you may have heard about preimplantation genetic testing (PGT). This advanced procedure helps screen embryos for specific genetic conditions before they’re implanted in your uterus. PGT can increase your chances of a healthy pregnancy by identifying embryos that have the correct number of chromosomes or those free from certain inherited disorders. It’s often recommended if you or your partner has a known genetic condition, a history of miscarriage, or is of advanced maternal age. The process involves creating embryos through IVF, then testing a few cells from each embryo. The cells are checked for any abnormalities, which helps your doctor choose the healthiest embryos for transfer.
Although it provides peace of mind and lowers some risks, it doesn’t guarantee pregnancy success. It's important to talk with your fertility specialist about the benefits, limitations, and costs to decide if it’s the right choice for you.
How Preimplantation Genetic Testing Is Performed on Embryos During IVF
Preimplantation genetic testing (PGT) is a specialised procedure used during IVF to screen embryos for genetic or chromosomal abnormalities before they are transferred to your uterus. It helps identify embryos with the best chance of resulting in a healthy pregnancy. The IVF process begins with the retrieval of your eggs, which are then fertilised with sperm in a lab to create embryos.The blastocyst is the early development stage of an embryo and consists of two cell types. The trophectoderm (TE) helps in the formation of the placenta, while the inner cell mass (ICM) eventually develops into the baby. By day 5 of the blastocyst stage, a biopsy is performed to extract 3-10 cells from the trophectoderm. These cells are then tested in the lab for genetic disorders. The results are usually available within 7 to 10 days after the biopsy.
The blastocyst is frozen immediately after the biopsy to await the test results. Once the results are available, the embryo is thawed and transferred to the gestational parent in a later cycle.
Does Preimplantation Genetic Testing Guarantee a Healthy Baby?
No, PGT provides valuable benefits, but it’s not foolproof. The testing only screens for specific conditions or abnormalities, meaning it does not guarantee a healthy pregnancy or baby.PGT adds cost and time to the IVF process. After testing, not all embryos may be suitable for transfer. It is important to discuss these factors with your fertility specialist, who can provide insight into the procedure’s accuracy, risks, and limitations based on your medical history.
Preimplantation Genetic Testing for Aneuploidy (PGT-A)
The primary purpose of PGT-A is to screen embryos for whole chromosome abnormalities. Aneuploidy means an embryo has an incorrect number of chromosomes, either too many or too few. This can lead to miscarriage, failed implantation, or conditions such as Down syndrome. PGT-A helps identify embryos with the normal 46 chromosomes, increasing your chances of a successful pregnancy.Humans have 46 chromosomes in total, arranged in 23 pairs. Monosomy is when a chromosome is missing, and trisomy is when there is an extra chromosome. A girl child can only survive one type of monosomy, called Turner syndrome, where one of the X chromosomes is missing.
Trisomy can sometimes lead to a live birth, such as with Down syndrome, where there’s an extra chromosome in pair 21. Down syndrome affects about 1 in every 830 live births in India. Other conditions include Turner syndrome and Patau syndrome.
Aneuploidy is one of the main causes of failed embryo implantation and miscarriage. It is also a significant cause of birth defects in children.
Who Should Consider PGT-A?
- If you’re over 35, your risk of chromosomal abnormalities in embryos increases. In such cases, PGT-A can be a useful option to consider.
- If you have had multiple pregnancy losses, PGT-A can identify embryos with the expected number of chromosomes to improve outcomes.
- If you have undergone several unsuccessful IVF cycles, PGT-A may help select viable embryos for better success.
- PGT-A may reduce the recurrence risk if you have had a pregnancy affected by conditions like Down syndrome.
Preimplantation Genetic Testing for Monogenic Disorders (PGT-M)
Preimplantation genetic testing for monogenic (PGT-M) is performed to screen embryos for specific single-gene disorders. If you or your partner are carriers of or affected by a known genetic mutation, PGT-M helps reduce the risk of passing these conditions to your child.A single-gene disorder results from a mutation in a specific part of the DNA sequence. PGT-M is used to detect embryos that carry known monogenic conditions such as cystic fibrosis, sickle cell anaemia, Huntington’s disease or Tay-Sachs disease.
Inherited genetic mutations, such as those in the BRCA1 and BRCA2 genes, can increase an individual’s risk of developing breast and ovarian cancer. PGT-M can help identify embryos that carry these mutations, reducing the risk of passing them on.
Preimplantation Genetic Testing for Chromosome Structural Rearrangement (PGT-SR)
PGT for structural rearrangements is a procedure used to screen embryos for chromosomal abnormalities caused by structural changes in the chromosomes. These chromosomal rearrangements, like translocations or inversions, can cause unbalanced chromosomes. This increases the risk of miscarriage, infertility, or genetic disorders in the baby.It identifies embryos with the correct number of chromosomes and balanced genetic material. Structural rearrangements occur when parts of chromosomes break and reattach incorrectly. If you or your partner carries a structural rearrangement, your embryos may inherit unbalanced genetic material, which can cause problems.
PGT-SR examines disorders such as:
- Reciprocal translocations
- Nonreciprocal translocations
- Robertsonian translocations
How Next-Generation Sequencing (NGS) Detects Mosaicism in Embryos
When embryos are created during IVF, doctors can test them for chromosomal issues before placing them in the uterus. This process, known as preimplantation genetic testing, includes advanced methods like next-generation sequencing (NGS). It uses molecular tools and advanced computing to look for abnormalities in chromosomes.Earlier, NGS results only told whether an embryo was normal (euploid) or abnormal (aneuploid). But now, NGS can also detect a condition called mosaicism. Mosaicism means that the embryo has a mix of normal and abnormal cells. This usually happens at the blastocyst stage, when the embryo has more than 100 cells. In a low-level mosaic embryo, most of the cells are normal, whereas in a high-level mosaic, the majority of cells are abnormal.
NGS has made it easier to detect mosaicism with better accuracy. This has led to the finding of mosaicism in as many as 20 per cent of embryos tested. Mosaicism may result from many different factors, and some embryos with mosaicism can still grow into healthy babies.
Traditionally, mosaic embryos were not chosen for transfer because it was believed they wouldn't develop into healthy pregnancies. But, a study titled ‘Healthy babies after intrauterine transfer of mosaic aneuploid blastocysts’ states that some mosaic embryos can lead to live births. However, the success rates are slightly lower.
Potential Risks of Preimplantation Genetic Testing
There are no known health risks for children born after PGT. The process of handling the embryo, including its biopsy, freezing, and thawing, carries a small risk of damage. This can result in an embryo that fails to implant. On average, about 5 per cent of embryos tested by PGT are lost due to this damage.There is also a possibility of inaccurate PGT results since the test is not 100 per cent reliable. It’s recommended that patients undergo standard prenatal testing during pregnancy.
Ethical Concerns Related to Preimplantation Genetic Testing
PGT is primarily used to screen for serious genetic disorders, but it could be used for non-medical traits in the future, like eye colour or intelligence. This raises concerns about eugenics and societal pressure to create “perfect” babies.Deciding which embryos to transfer or discard based on genetic results can feel like assigning worth to potential life. You might wonder if it’s fair to discard embryos with abnormalities, particularly when their viability is uncertain, as with mosaicism.
It’s important to remember that these decisions are deeply personal, and it’s okay to seek support to help you through the process.
Preimplantation genetic testing (PGT) can offer peace of mind by identifying healthy embryos before transfer. By screening for specific genetic and chromosomal issues, PGT allows you to make more informed decisions. However, it does not guarantee a successful outcome. It is important to consult a fertility specialist to decide if this approach suits your needs and circumstances.
FAQs
- Does preimplantation genetic testing guarantee a healthy baby?
No, PGT helps identify embryos with a lower risk of genetic conditions, but it does not guarantee a healthy pregnancy or baby. - What types of conditions can preimplantation genetic testing screen for?
PGT can screen for aneuploidy, monogenic disorders, and structural chromosomal rearrangements.
