Eggs are retrieved and fertilized with sperm in the laboratory.2. Embryo Cultivation to Blastocyst Stage:
The embryos are cultured in the laboratory until they reach the blastocyst stage.3. Embryo Biopsy:
On day 5–6 (sometimes day 7), a biopsy is performed at the blastocyst stage. The embryologist extracts a few cells (usually 5–8) from the trophectoderm layer (which will develop into the placenta) for genetic analysis.
4. Genetic Testing:
The extracted cells, also known as a biopsy sample, are tested for genetic abnormalities, including chromosomal disorders, monogenic diseases, or even the embryo’s Rh factor to avoid potential Rh incompatibility.
5. Embryo Selection:
Following the results of preimplantation genetic testing (PGT), a chromosomally healthy (euploid) embryo is selected for transfer.
Classification of PGT
PGT-A: Preimplantation Genetic Testing for Aneuploidy
This modern method identifies chromosomal abnormalities in the embryo before uterine transfer, offering several benefits:
- Reduces the number of unsuccessful transfers.
- Lowers the risk of miscarriage.
- Increases pregnancy chances for high-risk patients.
- Decreases the risk of multiple pregnancies.
- Minimizes the likelihood of chromosomal abnormalities in the child.
Why Perform PGT-A?
Chromosomal abnormalities can occur spontaneously in healthy individuals. With maternal age, the percentage of embryos with chromosomal abnormalities increases, significantly reducing the success rate of IVF without genetic testing.
Chromosomal abnormalities (both numerical and structural) occur in 48–90% of embryos depending on the reproductive age. Aneuploid embryos often appear indistinguishable from healthy ones under a microscope.
Testing focuses on common chromosomal disorders, such as Down syndrome, Patau syndrome, Edwards syndrome, Turner syndrome, and Klinefelter syndrome. At our clinic, PGT-A involves analyzing all 23 chromosome pairs, enabling the selection of a chromosomally healthy embryo for transfer.
When to Perform PGT-A?
PGT-A is recommended for:
- Couples with a family history of chromosomal disorders.
- Women aged 35 or older.
- Couples with prior unsuccessful IVF attempts.
- Male partners aged 40 or older.
- Cases of poor sperm morphology.
- Couples with a history of early pregnancy loss.
PGT-M: Preimplantation Genetic Testing for Monogenic Disorders
PGT-M identifies the genetic status of an embryo for specific monogenic disorders before uterine transfer. It is used for patients at high risk of transmitting certain genetic conditions to their children.
Who Should Consider PGT-M?
PGT-M is recommended when there is a high risk of transmitting specific genetic defects. This includes cases where:
- You or your partner are carriers of the same autosomal recessive condition (e.g., cystic fibrosis).
- You are a carrier of an X-linked condition (e.g., Duchenne muscular dystrophy).
- You or your partner have an autosomal dominant condition (e.g., Huntington’s disease).
- There is a mutation associated with hereditary cancer syndromes (e.g., BRCA1 & BRCA2).
- A previous pregnancy or child was affected by a genetic mutation.
PGT-M testing requires a detailed analysis of the mutation and the surrounding chromosome region. Each test design is unique and tailored, often requiring DNA samples from both partners.
PGT-SR: Preimplantation Genetic Testing for Structural Rearrangements
Structural chromosomal rearrangements involve changes in the size or structure of chromosomes, which carry genetic material.
What Can PGT-SR Detect?
PGT-SR is performed for couples where chromosomal rearrangements are identified in the karyotype, such as:
- Translocation: A chromosome segment shifts to another chromosome.
- Deletion: A chromosome segment is missing.
- Duplication: A chromosome segment is duplicated.
If the rearrangement is balanced (i.e., does not manifest in the individual), it can still affect gametes and be passed to offspring. This may result in embryos with significant defects, leading to miscarriage or a child with chromosomal abnormalities (unbalanced rearrangements).
How Is Testing Conducted?
During preimplantation genetic testing for structural chromosomal abnormalities, a biopsy is taken from a blastocyst (day 5–7 of development), and the DNA is analyzed using:
- NGS (Next Generation Sequencing): Detects extra or missing chromosomes.
- Karyomapping: Identifies balanced anomalies even when the chromosome count is normal.
Chromosomal rearrangements can be hereditary or spontaneous. Many carriers of balanced rearrangements are healthy and unaware of their status until they attempt to conceive.
Conclusion
PGT is an essential step in IVF treatment that significantly enhances its effectiveness, reduces the time to conception, and ensures the birth of a healthy child.