Pre-implantation genetic testing (PGT) are tests that evaluate the embryo before it is transferred to the uterus, thus increasing the success of in vitro fertilization (IVF).
Dr Raajam Murali, senior consultant and Dr R Suchindra, senior consultant, reproductive medicine, Milann Fertility Centre, Bengaluru, elaborate on the process, types and indications of PGT.
What is pre-implantation genetic testing?
Pre-implantation genetic testing (PGT) is a recent development in the field of artificial reproductive technology or IVF treatment. It tests the genetic profile of the embryo before it is implanted into the uterus, thereby increasing the chances of having a healthy baby.
What are the indications of PGT?
PGT is indicated in advanced maternal age, for those with an increased risk of miscarriage and high prevalence of aneuploidy (a genetic disorder where the total number of chromosomes doesn’t equal 46 and affects the outcome of the pregnancy) of oocytes and embryos. The other indications include couples with recurrent pregnancy losses, recurrent implantation failures and male partners with severe male factor infertility. PGT is also carried out if one or both parents carry a known chromosomal abnormality or carry the genes for single gene disorders, couples with a child or a family member with a serious genetic condition.
How is PGT done?
To do PGT, the couple must undergo an IVF cycle (in vitro fertilization). It will begin on day 2 or day 3 of the menstrual cycle. It starts with scans and daily injections for about ten days. Serial scans are done to monitor the growth of the egg. Transvaginal aspiration of the eggs will be done. Egg retrieval will be done under sedation and the eggs will be inseminated with the male partner’s sperm to form embryos.
The embryos will be cultured in the lab for about five days till they reach the blastocyst stage. Once the blastocyst expands, a few cells from the outer layer of the blastocyst called trophectoderm are removed by biopsy and sent for testing. The scientist will run tests on the embryo using the next-generation sequencing technique and report it as euploid or aneuploid.
If the number and structure of chromosomes of the embryo cells are normal, it is termed euploid. If the number and structure of the chromosomes of the embryo are abnormal, the embryo is called aneuploid. Aneuploid embryo will not implant and will result in a negative pregnancy test or miscarriage. Euploid embryos have higher chances of implanting and resulting in a positive pregnancy test, increasing the success of an IVF program.
The euploid embryos are selected and implanted into the uterus and aneuploid embryos are discarded with the couple’s consent.
The couple will undergo genetic counselling before PGT where they are explained the procedure, the risks and benefits after which they can give consent to undergo the test.
Pre-implantation genetic testing comes under the Preconception and Prenatal Diagnostic Testing Act of India which prohibits sex selection. So, sex chromosomes are not revealed in this test.
What are the different kinds of PGT?
There are two types of PGT. One is PGS (pre-implantation genetic screening) done on couples who are genetically normal. PGD is done (pre-implantation genetic diagnosis) in couples who have genetic issues in their family or have karyotype with chromosomal translocations (a genome abnormality in which a chromosome breaks and either the whole or a portion of it re-attaches to a different chromosome)
By doing PGD, we prevent genetic diseases from transmitting from the parents to the children, thereby ensuring a healthy child is born.
Does PGT increase the success of IVF?
Abnormal or aneuploid embryos lead to implantation failure and miscarriages, thus resulting in unsuccessful IVF.
Transferring chromosomally normal or euploid embryos increases the chances of conception. It also reduces the abortion rate and time for conception in IVF cycles.
How successful is IVF without PGT?
The success rate of any IVF program depends on the age of the mother. As age increases, fertility reduces and the success of an IVF program also reduces as the woman becomes older.
In women below 35 years, the success rates of an IVF program will be around 50 to 55 percent. Between 36 to 38 years, it falls to 40 to 45 percent. From 38 to 40 years, it’s about 30 to 35 percent. Above 40, it’s about 20 percent, above 42, it’s less than 10 percent.
So, by doing PGT in women above 38 years, you select the embryo, which is euploid, that is, without chromosomal problems so that it increases the success rate of older women.
So, if a woman above 38 years undergoes PGT testing and transfers her embryos, her success rate will be that of a woman who is less than 35 years.
What age is most appropriate for PGT and IVF?
PGS is offered to women who are more than 38 years old to improve their success rate. It can be done in those less than 35 years if they have multiple IVF failures, recurrent implantation failures, repeated abortions, women who have genetic diseases in the family or women who have HLA (human leukocyte antigen) incompatibility. Men with severe infertility can opt for PGT even at a younger age.
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