Instituto BernabeuMiscarriage 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. 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.

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 Comprehensive Chromosome Screening (CCS) 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. Until relatively recently, PGS/CCS was performed using the array CGH technique that consists of comparing the embryo’s DNA with a control DNA. Chromosomal excesses or deficiencies in the embryo can be detected in this way and transfer can, therefore, be avoided. However, since the incorporation of next generation sequencing (NGS) techniques to PGS (PGT-A), most of the embryos in our clinic are analysed using NGS because, in comparison with array CGH, this technique can be used to analyse multiple embryos with increased precision in diagnosis. This further decreases the likelihood of the embryo having a chromosomal abnormality and leading to pregnancy loss.

At Instituto Bernabeu we incorporate this state-of-the-art technology to guarantee the best results to our patients.

Dr. Ruth Morales, molecular biologist at Instituto Bernabéu.

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Why does Comprehensive Chromosome Screening (PGS/PGT-A/CCS) by array-CGH reduce miscarriage rates?
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