PGT: What is it and why does it matter?

While Preimplantation Genetic Testing sounds like a mouthful, PGT is actually a very helpful tool and isn’t quite as complicated as it might seem.

Okay, so what is PGT?

PGT is a diagnostic tool that lets genetics labs study the DNA of embryos for particular conditions before they are transferred to the uterus. This peek into an embryo’s genetic makeup can help your doctors prevent transferring embryos that have certain inherited genetic disorders or abnormalities. Both of which may compromise the likelihood of pregnancy, or even the baby’s health down the road.

PGT is an option for everyone* who goes through the IVF process. The study is done by making a hole in the outer layer of an embryo 3-5 days after fertilisation. One of the cells is removed through this opening to be studied. The study will tell the embryologist whether it is normal or abnormal, and the normal ones can be transferred.

There are multiple tests to look at the genetics of an embryo. Clinicians will be able to guide you to the right technique for you, but let’s look at a few of the most common types.

What about aneuploidy screening?

The growth of an embryo into a baby is guided by its chromosomes, and aneuploidy screening, or PGT-A, helps make sure embryos have all their chromosomes when they’re just 5 or 6 days old. Even at this very early stage, each normal embryo contains 46 chromosomes. 23 are contributed by the sperm and 23 by the egg. An embryo that contains 46 normal chromosomes is called a euploid embryo. An embryo that has more or less than 46 chromosomes is called an aneuploid embryo. An aneuploid embryo is an embryo that is missing or has extra chromosomes. Unfortunately, these embryos often fail to implant, and those that do, generally result in miscarriage. In cases where aneuploid embryos do develop to term, they can often have major health concerns, the most common being Down’s Syndrome.

Once the genetic testing has confirmed that the embryo is normal and has the correct number of chromosomes (there should be two copies of each), it can be selected for transfer. Knowing that the embryo is normal beforehand increases the overall odds that the transfer results in a healthy pregnancy and delivery, which is always what we hope for.

PGT-A might be the right option for you if you’ve had repeated transfers that didn’t result in pregnancy, multiple miscarriages, or if you’re over 36 years old.

What is a chromosomal rearrangement?

As we know, embryos should have 46 chromosomes, 23 from the egg and 23 from the sperm. They should be laid out in sequential order as well. Chromosomal rearrangements occur when there is a difference in the normal size or order of chromosomes. Sometimes, one or both partners might have a chromosomal rearrangement themselves, which means it’s very likely they can pass that on to their children. Chromosomal alterations can also cause miscarriage, birth defects, or other health concerns.

With this study, however, PGT enables us to see and select embryos that are chromosomally normal and discard those that are chromosomally unbalanced. Once the treatment has been given and the embryos have been studied, your clinician will be able to transfer embryos that are normal for the rearrangement in question, increasing the likeliness of a healthy pregnancy and baby,

When would I use PGT for sex selection?

While the egg and sperm each contribute 23 chromosomes to the embryo, sometimes there are certain severe X-linked recessive diseases that cause the egg to pass down the disease and males to be affected by it.

In these cases, PGT is performed to distinguish male embryos (XY) from female embryos (XX) and transfer only those that are female (XX). This makes it possible to transfer embryos that will not develop the disease even though they will be carriers. Although there are many such diseases, the most well-known X-linked diseases are:

  • Haemophilia
  • Duchenne muscular dystrophy
  • Lesch-Nyhan syndrome
  • Hunter syndrome

How can PGT help me with monogenic disorders?

Nowadays, PGT can identify a wide range of serious genetic disorders caused by mutations in the embryo prior to transfer. By only transferring embryos without the disorder, it can be guaranteed that the disease in question will not be passed on to subsequent generations. The basis of this test is the same as in other PGTs, and includes an informativity study, embryo screening, and transfer of healthy embryos. The diseases for which PGT is most commonly performed are:

  • Cystic fibrosis
  • Huntington's disease
  • Fragile X syndrome
  • Steinert's disease
  • Tuberose sclerosis
  • Polycystic kidney disease
  • Thalassemia

This is an open-ended list that will be updated as new information comes to light. Hopeful parents should consult their clinician to see if the disease affecting their family has been added to the list and determine whether they may benefit from advances made in genetic diagnosis.

  • Laws are different for genetic tests during fertility treatments around the world. This option may not be available in your country. But if it is important for you, you can consider travelling to a clinic in another country.