Pre-implantation Genetic Testing (PGT)
Providing an insight into the embryo's genetic health to maximise the chances of a successful pregnancy
The primary goal of fertility care is to provide couples with a successful pregnancy that results in a healthy child free of disease and genetic abnormalities. Pre-implantation genetic testing (PGT) is a screening test performed on embryos created via in vitro fertilsation (IVF) to genetically analyse the embryos prior to transfer. In many cases, PGT can allow your physician to select embryos for transfer predicted to be free of chromosome abnormalities or a specific genetic condition and ultimately increase your chances of having a successful pregnancy.
How pre-implantation genetic testing works
PGT involves doing a biopsy and removing a small number of cells from an embryo so that it can be screened for genetic abnormalities or conditions.
The biopsy is often done on day 5 or 6 embryos. At this stage of development, the embryo is called blastocyst and consists of an inner cell mass and an outer layer of cells, known as the trophectoderm. The inner cell mass will eventually develop into the fetus, while the cells from the trophectoderm will become the placenta. Biopsies are taken from the trophectoderm, and the cells that will form the baby are left intact and are not biopsied for testing.
After the biopsy the embryos are frozen awaiting the results of the PGT. The patient will be scheduled for a frozen embryo transfer cycle at a later date. PGT is a safe procedure and is offered by most state-of-the-art IVF clinics.
Who are good candidates for PGT?
Whilst in some clinical settings IVF with PGT is offered to most patients, PGT is usually offered for the following reasons:
The benefits of PGT
In general, PGT does not improve the pregnancy rate in IVF patients especially those under the age of 35. However, recent evidence suggests that it can lower the risk of miscarriage and increases the live-birth rate in women above 37 years of age. It can also reduce your risk of having a baby with chromosomal abnormalities.
Types of genetic testing
There are different types of genetic testing services that are used in different scenarios. When deciding whether or not to undergo PGT as part of your IVF cycle, your fertility specialist in some cases together with a genetic counsellor will work with you to select the best test that fits your history, clinical case and preferences.
PGT-A is the most common form of embryo testing. It tests whether an embryo has the correct number of chromosomes. The correct number of chromosomes (46), also known as euploid, is necessary for the embryo to become a normal child. When an embryo’s cells have an incorrect number of chromosomes, either too few or too many, it’s known instead as aneuploidy. It can also tell you whether an embryo is male or female (XY or XX).
The prevalence of aneuploid embryos increases as women get older. Embryos with aneuploidy are more likely to result in an early miscarriage or failed IVF cycle, irrespective of the morphological appearance. Less commonly, aneuploidy embryos may result in the birth of a child with a chromosome condition, such as Down syndrome (trisomy 21).
In addition to Down syndrome, PGT-A can detect other full chromosome genetic abnormalities such as Turner’s syndrome (XO) and Klinefelter’s (XXY). PGT-A has been reported to have a high accuracy rate of about 98%.
A PGT-A normal embryo is estimated to have a 60% chance of live birth from a transfer cycle.
PGT-M is used to confirm whether a specific inheritable condition is present within a patient’s embryos. It enables couples with an inheritable genetic condition in their family to avoid passing it on to their children. PGT-M is typically recommended in cases where one or both partners have a known history of genetic conditions in their families. Examples of some common single gene disorders that can be diagnosed with PGT-M include: Thalassemia, Hemophilia A and B, Fragile X syndrome, Sickle cell disease, Cystic fibrosis, Duchene muscular dystrophy, Huntingdon’s disease, and Marfan syndrome.
PGT-SR identifies embryos with the correct amount of genetic material (balanced/normal) and embryos that have extra or missing genetic material as a result of the translocation or rearrangement (unbalanced).
This test is the least common of them and most patients will not need this type of testing.
PGT-A for non-medical indications: Gender selection
PGT-A has allowed patients to know the sex of their embryos. Sex selection for medical reasons is used to prevent the birth of children affected by certain sex-linked genetic diseases. However, patients’ interest in selecting the sex of their child (non-medical sex selection) using PGT-A appears to be increasing. Indications for non-medical sex selection include “family balancing” (to have a child of the opposite sex to previous offspring) and personal preference (opting to have either a male or female child for the unique experience of raising a child of one sex or another). The sex selection process is >99% accurate.
The use of IVF and PGT-A for non-medical sex selection has been the subject of continuous debate amongst the public and professionals. Although non-medical sex selection is prohibited in many countries, in some countries it is allowed and acceptable.
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