The advantages and disadvantages of pre-implantation genetic diagnosis (PGD)

Pre-implantation genetic diagnosis (PGD) consists of studying chromosomal abnormalities and genetic abnormalities in the embryo prior to transfer to the mother. Its purpose is to ensure that children are healthy and put an end to the transmission of a specific condition.

There are two types of PGD: the PGD aimed at selecting embryos that are free of a genetic disorder affecting a single gene (PGD) and the PGD that analyses genetic diseases affecting one or more chromosomes (PGS/PGT-A/CCS or PGS). Their names are sometimes a cause for confusion. The post entitled Are PGD, PGS and CCS all one and the same? clarifies the differences between them.

In the first instance, there is a clear advantage to the use of the PGD that studies the genetic diseases affecting a single gene and its use is not questioned. It is the only means of avoiding transmission of a genetic disorder without the partner having to face the possibility of the child she is carrying being affected and, therefore, having to face the decision of terminating the pregnancy.

There is, however, some disagreement regarding to what degree the use of PGD aimed at studying chromosomal disorders (PGS) improves success rates in assisted reproduction. The main advantages and disadvantages of both points of view are explained below.

ADVANTAGES:

IMPROVED EMBRYO SELECTION

Only embryos presenting no abnormalities in terms of the number of chromosomes will lead to the birth of a healthy child. Therefore, when working with good embryos, when PGS techniques are applied, we are able to select chromosomally normal embryos and rule out those which would never be capable of leading to the birth of a healthy child even when their appearance would suggest that they are good quality embryos.

AVOIDS THE TRANSFER OF EMBRYOS THAT WILL NOT IMPLANT

Certain chromosomal abnormalities are incompatible with life and prevent the embryo from developing during its early stages and even from implanting in the mother’s uterus. PGS means that embryos of this kind can be ruled out, thus optimising the number of transfers.

AVOIDS THE TRANSFER OF EMBRYOS THAT WILL LEAD TO PREGNANCY LOSS OR THE BIRTH OF CHILDREN WITH A VARIETY OF SYNDROMES

Within the range of possible chromosomal abnormalities, some are less harmful to the embryo and allow it to implant. However, they do stop the pregnancy from developing correctly and can lead to pregnancy loss or the birth of a child with a number of possible syndromes such as Down’s syndrome, Patau’s syndrome or Edwards’ syndrome. PGS means that embryos that will be the cause of situations of this kind can be ruled out.

REDUCES THE TIME REQUIRED IN ORDER TO GET PREGNANT

By using PGS, we are able to avoid transferring embryos that will not lead to the birth of a healthy child since they will have been ruled out using the technique. Since we know which embryos will give rise to a full term pregnancy, ‘time is not wasted’ transferring embryos that will undergo embryo arrest during development and will not lead to the birth of a healthy child.

REDUCES THE FINANCIAL BURDEN

Adding a new analysis to the process could be indicative of an increase in the cost. However, an in-depth knowledge of the characteristics of each embryo means that embryos that would appear to be healthy but which, in fact, are not healthy from a genetics point of view are not frozen and stored. Additionally, the cost of transferring embryos that will not lead to a pregnancy is avoided.

POSITIVE IMPACT ON PSYCHOLOGICAL WELLBEING

Using PGS means that the uncertainty that patients go through is reduced. On the one hand, they have the guarantee of the health of their embryo and that the very latest technology has been used in order to ensure this. On the other, the risk of pregnancy loss is reduced and this reduces emotional stress, particularly in the case of patients who have already gone through this.

DISADVANTAGES:

INVASIVE PROCEDURE

PGS means that the embryo needs to be biopsied in order to carry out the genetic test. However, significant progress to reduce the possible damaging effect of the biopsy has been made over the last few years. Carrying out the embryo biopsy on day 5 of development rather than on day 3 has been key to now being able to say that the embryo biopsy does not have a negative impact on embryo viability.

A CYCLE WITH NO TRANSFER

In some cases, patients are at a high risk of having abnormal embryos. This is the case, for example, of mothers of an advanced age. When this is the case, it is possible that, following PGS analysis, all the embryos are chromosomally abnormal and not suitable for transfer. As well as the upset caused by calling the treatment off, there is also a significant emotional impact.

EMBRYO MOSAICISM

It is commonly accepted that human embryos have a certain degree of mosaicism. However, diagnosis was difficult. Nowadays, thanks to the development of genetic analysis techniques, we are able to see if there are both normal and abnormal cells in the embryo (mosaic). What needs to be determined is if this affects the embryo in any way. Numerous lines of research work have been carried out at Instituto Bernabeu in order to evaluate this.

PGS AS A MEANS OF SCREENING

PGS analyses the outer section of the embryo with the aim of leaving the part that will give rise to the baby (the internal cell mass) intact because scientific research has shown that there is a significant connection between the two. Therefore, we accept that the biopsy sample taken is representative of the entire embryo.

A DIFFICULT DECISION

Many couples find taking the decision to analyse their embryos difficult for ethical and emotional reasons. Psychological and professional support is available to patients at our clinics. This serves as a guide but the final decision is always up to the family.

Having evaluated the advantages and disadvantages of PGS, we can conclude that there is a clear advantage to using PGS in assisted reproduction techniques since it IMPROVES THE SUCCESS RATE OF TREATMENT.

At Instituto Bernabeu, we have a team of staff and the very latest technology in order to carry out this technique in our own facilities. This means we are able to offer patients the very best quality standards in the results obtained whilst decreasing the time taken and auditing the entire process. We also have a unit that specialises in Genetics and Reproduction Counselling aimed at couples for the duration of the process and that provides a personalised analysis of each case.

Dr Belén Lledó, Scientific Director at IBBIOTECH, part of the Instituto Bernabeu group.

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