Pre-implantation Genetic Diagnosis (PGD)

Some couples are at risk of having a child with a specific serious genetically inherited disease. Such a disease may be due to an abnormality affecting the genes, chromosomes or tiny structures within the cell called mitochondria. You may have become aware of this because:

there is a family history of a genetically inherited disease;
you have a child with a genetically inherited disease;
you have had terminations of pregnancy for a genetically inherited disease.

Until recently, pre-natal testing (amniocentesis and chorionic villus sampling) offered the only means of diagnosing serious genetic conditions affecting the baby. When found, the distressing option of having the pregnancy terminated could then be considered.

PGD provides an alternative to prenatal screening as embryos can be tested for any one of over 50 serious inherited disorders.

PGD checks the genetic make up of embryos obtained through IVF so that only unaffected embryos are transferred. Through PGD couples are able to avoid passing on the condition to their children.

The HFEA will only give a licence to test an embryo for a specific genetic abnormality if:

that genetic abnormality may affect an embryo?s capacity to result in a live birth;
a child born with that genetic abnormality may have or may develop a serious physical or mental disability or a serious illness.

If your clinic does not have a licence to test your embryos for the particular genetic disorder in your family, the clinic must apply for such a licence from the HFEA before you can commence treatment.

There are some conditions where a serious genetic abnormality will only affect one sex. These are known as sex-linked disorders. Duchenne Muscular Dystrophy is one such condition which only affects the male. The woman can carry the gene that causes the disease bur she is not affected by it. In this situation a licence can be granted to allow sex selection to determine the sex of embryos and only select female embryos for transfer (see Pre-Implantation Genetic Screening (PGS) information).

It is not permitted to use sex selection for social reasons to try and achieve a balanced family.

It is not permitted to deliberately select an affected embryo for transfer in preference to an embryo that is known not to carry such a risk.

There are some genetic diseases inherited by children that are life threatening and can only be treated by using stem cells from a close member of the family. The problem here is that members of the same family may not have the same tissue type and a suitable donor cannot be found. In this rare and very distressing situation, PGD can offer the parents of such a child the opportunity of having a baby whose tissue type is an exact match to that of the existing child. Stem cells taken from the umbilical cord of this baby can be used to treat the affected child. This is a very emotive area of PGD and sensitive counselling is essential. The "rescue" baby is not being brought into existence simply as a bank of stem cells for its older sibling, but must be loved and wanted in its own right. There are understandable concerns that later in the life of this child, pressure may be brought to bear to donate other tissues or even organs as a life-saving measure.

PGD involves a multidisciplinary team, to include the clinic's own reproductive medicine specialists, embryologists trained and approved by the HFEA to carry out the necessary embryo biopsies, clinical genetics specialists, scientists and counsellors.

How is PGD performed?

IVF is carried out in the normal way (see IVF information). By day 3 the embryo will contain 6-8 cells (blastomeres). The embryologist removes 1-2 blastomeres for testing.

Embryos that are found to carry the faulty gene that causes the specific inherited disease are allowed to perish.

1-3 embryos that are free the genetic disorder can then be transferred. If there are any suitable additional tested embryos, these can be stored for future use by freezing (see Cryopreservation of embryos information).

Success rates of PGD

In 2005, for women receiving PGD, the percentage of cycles started that resulted in a live birth was:

12.5% (8/64) for aged women under 35
**(6/35) for women aged between 35-37
**(1/8) for women aged between 38-39
**(1/14) for women aged between 40-42
**(0/4) for women aged between 43-44
No PGD cycles were carried out for women aged over 44

** Percentages are not calculated where there are less than 50 cycles. Figures given in brackets are (cycles resulting in a live birth / all cycles started)

Figures provided by courtesy of the HFEA

What are the problems of having PGD?

You will be going through an IVF cycle and the risks linked to such treatment (see IVF information).

PGD is expensive. It is important that you appreciate all costs in advance of commencing treatment.

You must also be aware that it is possible that all of the embryos that are tested contain the genetic abnormality and that no embryos from that particular IVF treatment cycle will be available for transfer. Occasionally the blastomere biopsy can damage the embryo making it unsuitable for transfer.

March 2009