A Clear Understanding on Preimplantation Genetic Testing
Enhancing IVF Success Through Embryo Screening
8A.
Transfer to Uterus
Unaffected (normal) embryos are thawed and transferred to the uterus for potential pregnancy.
Preimplantation genetic testing (PGT) is an established reproductive option for couples with a risk of conceiving a baby that is affected with some known genetic disease. The aim of PGT is to have an unaffected child and in turn avoiding the trauma of terminating the pregnancy. This method allows embryo testing for embryos being produced via in vitro fertilization (IVF) before transferring the embryo into the uterus.
Types of PGT
There are two types of genetic testing of an embryo prior to transfer:
Preimplantation Genetic Screening (PGT-A) and
Preimplantation Genetic Diagnosis (PGT-M).
Preimplantation Genetic Diagnosis (PGT-M).
PGT-A
Preimplantation genetic testing for aneuploidy (PGT-A) serves as a screening procedure to identify embryo with chromosomal abnormality, which is thought to be the most common reason IVF treatment fails. For example, Down syndrome is the most common chromosome disorder that we know of which carries an additional chromosome 21.
Genetic testing for abnormal chromosome number is known as Preimplantation Genetic Testing for Aneuploidies (PGT-A).
Indications for PGT-A are :-
- Older women who are trying to conceive
- Recurrent miscarriages
- Achieving live birth at the shortest time
So, is PGT-A recommended for every couple regardless of women’s age?
The answer is a big resounding NO. The data collected by Society of Assisted Reproductive Technology (2015) revealed that PGT-A is only helpful for women aged 35 years and above which further proves that women who are older have lower chance of pregnancy and/or delivering a healthy baby.
PGT-M
In contrast, Preimplantation genetic testing for monogenic/single gene disorder (PGT-M) refers specifically to when one or both genetic parents have a known genetic abnormality and testing is performed on an embryo to determine if it also carries the genetic abnormality.
Because only unaffected embryos are transferred to the uterus for implantation, PGT-M provides an alternative to current post-conception diagnostic procedures (i.e. amniocentesis or chorionic villus sampling), which are frequently followed by the difficult decision of pregnancy termination if results are unfavourable. PGT-M is presently the only option available to avoid having a child affected with a genetic disease prior to implantation. It is an attractive means of preventing heritable genetic disease, thereby eliminating the dilemma of pregnancy termination following unfavourable prenatal diagnosis.
Genetic testing to diagnose embryos with known genetic disorder is now known as Preimplantation Genetic Testing for Monogenic Disorder (PGT-M).
if a man and/or woman has a known genetic disorder (eg : Thalassemia) and are trying to conceive, they can opt for PGT-M to reduce the risk of pregnancy with the inherited genetic disorder.
How is PGT performed?
PGT begins with normal In vitro fertilization (IVF) that involves egg retrieval and fertilization in the laboratory. The embryos are cultured until it reaches a stage called blastocyst, which normally occurs either fifth or sixth day post egg collection. At this stage of development, a few cells are removed from it. The genetic material (DNA) of the biopsied cells are then assessed and verified if the problematic gene is present in each embryo. After cell biopsy, the blastocysts are frozen until results of the test are made available. Once the normal embryos are identified, they are safe to be transferred to the uterus. The surplus unaffected embryos will be kept frozen for future use while the affected embryos are discarded.
1. Ovarian Stimulation & Egg Retrieval
Hormones are given to stimulate the ovaries. Eggs are retrieved from the ovaries.
2. FERTILIZATION (IVF)
Retrieved eggs are fertilized with sperm in the laboratory to create embryos.
3. EMBRYO CULTURE TO BLASTOCYST
Embryos are cultured in the lab until they reach the blastocyst stage (day 5 or 6 post egg collection).
4. Biopsy
A few cells are removed from the trophectoderm (outer layer) of the blastocyst.
5. Genetic Testing
DNA from the biopsied cells is analyzed to determine whether the problematic gene is present in each embryo.
6. Cryopreservation (After Biopsy)
After biopsy, blastocysts are frozen (vitrified) while waiting for test results.
7. Results & Embryo Selection
Once test results are available, embryos are classified as :
- Unaffected (normal)
- Affected (abnormal)
8. Embryo Disposition & Transfer
8A.
8B.
Store for Future UseSurplus unaffected (normal) embryos are kept frozen for future use.
8C.
DiscardAffected (abnormal) embryos are discarded.
Talk to your Fertility Specialist if you are unsure which genetic testing is suitable for you.