Why does the PGT-A preimplantation genetic test (PGT) decrease the miscarriage rate?

Why does the PGT-A preimplantation genetic test (PGT) decrease the miscarriage rate?

Miscarriage rates in the general population with no fertility problems range around 15-20%. In other words, one out of every five couples who achieve pregnancy suffers a spontaneous miscarriage, and 5% of these couples suffer it more than once. Even when pregnancy is achieved with the help of assisted reproduction techniques, miscarriage rates do not vary. For this reason, it is important when couples come to our clinic seeking reproductive counselling to perform comprehensive testing and design an adequate protocol for their case to secure the best result, which is a healthy baby at home, minimising the chances of miscarriage. In order to do that, one should know that miscarriages and pregnancy losses are caused by different reasons (uterine problems, immunological problems, and so on), and yet in half the cases there are chromosomal abnormalities in the embryo that prevent pregnancies to progress, hence causing miscarriages.

Role of genetics in miscarriages

Normal embryos have two copies of each chromosome, one inherited from the father and the other from the mother, and the chromosomal anomalies they may suffer involve a change in the number of copies, producing an imbalance in their genetic load which might block embryo development.

The high rates of chromosomally abnormal embryos are due to:

  • On the one hand, the high rates of chromosomal abnormalities that human gametes (oocytes and sperm) have, which can be transferred to embryos. This rates increases as women age, especially from age 35. 40-year-old women have a high number of abnormal oocytes and, therefore, little chance of having healthy embryos.
  • On the other hand, the fact that these abnormalities can also occur spontaneously during embryo splitting.
  • Finally, a chromosomal alteration in the karyotype in either the male or the female will result in a percentage of chromosomally abnormal embryos.

What is PGT-A preimplantation genetic testing (PGt)?

In assisted reproduction techniques (IVF or ICSI), assessing embryo morphology does not suffice in order to determine if the embryo is chromosomally normal. As a consequence, the results are worse than expected, given that chromosomally anomalous embryos, which do not implant and cause miscarriages, are also transferred. The recent development of preimplantation genetic aneuploidy test or PGT-A has been a useful tool to determine the chromosomal state of the embryos produced by these techniques prior to being transferred to the mother’s uterus, thus preventing pregnancy losses for chromosomal reasons and, therefore, reducing miscarriage rates and increasing the rates of babies at home. The PGT-A, through the biopsy of the embryo, allows the detection in gains (trisomies) or losses (monosomies) of any chromosome avoiding the transfer. With the recent incorporation of mass sequencing techniques (NGS) to preimplantation genetic diagnosis (PGT-A) as well as the performance of the biopsy of the embryo on the day 5 or 6 of its development, the accuracy of the genetic diagnosis of the embryo has been improved. Therefore, PGT-A is now a highly reliable and safe diagnostic technique routinely used in in vitro fertilization processes.

Cases in which the PGTA is indicated (PGT)

PGT-A would be indicated in cases with high risk of chromosomal abnormalities in the embryo and therefore a high risk of miscarriage, such as:

  • Male or female with a chromosomal alteration in their karyotype
  • Women of advanced maternal age (40 years or older)
  • Couples who suffered two or more miscarriages
  • Males who have a high percentage of chromosomal abnormalities in their sperm (altered FISH).

At Instituto Bernabeu we incorporate this state-of-the-art technology. Therefore we perform this type of diagnosis in the couples when indicated, thus reducing the probabilities of miscarriage, and guaranteeing the best results to our patients.

Dr. Ruth Morales, molecular biologist at Instituto Bernabéu.

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