Preimplantation Genetic Testing for Aneuploidy (PGT-A)

Chromosomes are genetic structures found in nearly every cell of our bodies. The first cell of an embryo is made up of a set of chromosomes from the sperm and another set from the egg. But if the cell doesn’t have the right number of chromosomes, known as aneuploidy, this can lead to implantation failure, miscarriage or a baby affected with a serious condition.

PGT-A is a technique that allows selection of embryos with normal chromosomes from a group of embryos. After ICSI, a few cells from each embryo are sent to the genetics lab and chromosomes are counted. The embryos with a normal number of chromosomes are selected from the group for transfer.

Embryo selection by conventional imaging techniques is not very selective. Good looking embryos are not always good quality embryos. Abnormal embryos often look just like normal embryos under the microscope. When an abnormal embryo results in pregnancy, miscarriage is possible. To compensate, IVF programs may transfer more than one embryo, risking multiple pregnancies.

Patients who can benefit from PGT-A include those:

  • Couples with a history of recurrent miscarriages;
  • Couples who have had several failed IVF attempts;
  • Couples who have had previous pregnancies affected by numerical chromosomal abnormalities;
  • Men with severe male factor infertility;
  • Women over the age of 35.
  • Risk of multiple pregnancies
  • Risk for sex-linked disease in offspring
  • Plans to use a gestational carrier

An embryo that contains a normal number of chromosomes is a 'euploid' embryo. An embryo that carries an abnormal number of chromosomes is 'aneuploid'.

Age is the best predictor of aneuploidy and a major factor to consider when thinking about aneuploidy. In a woman under age 30, about half of embryos are aneuploid. At age 42, around 80% of embryos are aneuploid.

Most of the aneuploid embryos don't grow, and are never seen clinically. A few aneuploid embryos continue development and result in clinical problems. Miscarriage occurs in about a fourth of pregnancies – most of these are aneuploid.

About 3 out of 4 of these euploid embryos will implant on average (75%  implantation rate), with high pregnancy rates per transfer.


PGT-A Technique:

The technique of PGT-A requires IVF and growing embryos to the blastocyst stage (day 5 to 7). The embryo is "hatched” in the IVF lab using laser, and a few cells sampled from the outer trophectoderm layer of cells (which later goes on to form the placenta and membranes of a pregnancy). The embryo is then vitrified (an advanced technique of embryo freezing), and stored for later transfer back to the uterus if euploid. The biopsied cell samples are sent to a PGT lab for a count of the chromosomes.

Embryos are stored after sampling, since genetic testing takes about 2 weeks. All embryos do well with the freezing-thawing process, with >97% of embryos surviving vitrification and thaw.

Expectations are that on average about 1/10 eggs produces one chromosomally normal embryo.



Advantages of PGT-A include:

  • Improved selection of embryos most likely to result in healthy pregnancy and live birth
  • Reduce the numbers of embryos transferred
  • Reduction in aneuploidy rates
  • Reduction in miscarriage rate
  • Improved implantation rate
  • Reduced multiple (twin or more) pregnancy risk
  • Reduced cryo preservation rates

Disadvantages of PGT-A include:

  • Rare errors in test results, assumed to be mostly due to mosaicism (mixture of euploid and aneuploid cells in an embryo) that is not detected
  • Data on safety incomplete, but reassuring
  • Data on delivered child outcomes is under continued study.

Evaluating PGT-A results: