Pre-implantation genetic testing

Pre-implantation genetic testing involves carrying out tests on embryos created through in vitro fertilisation (IVF) or intra-cytoplasmic sperm injection (ICSI) to detect certain inherited conditions or abnormalities.

This helps to ensure that only unaffected embryos are selected before they are transferred to the womb.

Conventional pre-natal tests for genetic diseases cannot be carried out until the 12th week of pregnancy.

Testing embryos before they are implanted could help you and your partner to avoid having to make the difficult decision whether to have an abortion if either of you is the carrier of a genetic condition and the embryo is affected.

 

What is PGD?

Pre-implantation genetic diagnosis (PGD) is a technique that enables people with a specific inherited condition in their family to avoid passing it on to their children. It involves checking the genes of embryos created through IVF for this genetic condition.

Is PGD for me?

Genetic testing of your embryos may be recommended if:

• you have had a number of abortions because your baby had a genetic condition 
• you already have a child with a serious genetic condition 
• you have a family history of a serious genetic condition.

Which genetic conditions can be tested for during PGD?

PGD can be used to test for over 100 genetic conditions. To see the types of conditions that may be tested for, see the HFEA’s list of PGD conditions:

 The HFEA must agree that a particular genetic condition is sufficiently serious before clinics are permitted to test for that condition using preimplantation genetic diagnosis (PGD).

Even if the HFEA approves the genetic condition for testing, clinics must make their own judgment on whether PGD is the appropriate treatment for a particular patient. In doing this, they will use the guidance contained in the HFEA’s Code of Practice. This guidance requires them to take into account the view of that patient of the seriousness of the condition to be avoided.

PGD is an area of medicine that is rapidly developing, so new tests often become available. If a condition is not approved by the HFEA, we recommend getting in touch with a licensed PGD clinic to see whether they are prepared to make an application for that condition to be approved.

How does PGD work?

The procedure for PGD is likely to be as follows:

Step 1. You undergo normal in vitro fertilisation (IVF) treatment to collect and fertilise your eggs.

Step 2. The embryo is grown in the laboratory for two - three days until the cells have divided and the embryo consists of around eight cells.

Step 3. A trained embryologist removes one or two of the cells (blastomeres) from the embryo.

Step 4. The cells are tested to see if the embryo from which they were removed contains the gene that causes the genetic condition in the family.

Step 5. Embryos unaffected by the condition are transferred to the womb to allow them to develop.

Step 6. Any suitable remaining unaffected embryos can be frozen for later use. Those embryos that are affected by the condition are allowed to perish.

Step 7. About two weeks after the embryo transfer, the woman is given a pregnancy blood test.

Trophectoderm biopsy
It is possible that, instead of removing and testing one or two cells from a 2-3 day old embryo, some centres may allow the embryo to develop to 5-6 days, when there are 100-150 cells.

At this stage, cells within an embryo have separated into two types: cells which will form the fetus (inner cell mass) and cells which will form the placenta (trophectoderm).

More cells can be removed at this stage (from the trophectoderm) without compromising the viability of the embryo, possibly leading to a more accurate test.

 

What is the chance of having a baby with PGD?

It is difficult to assess success rates for PGD because there is currently little data available. Most women use this treatment not because they have fertility problems but because they want to avoid having a child with a genetic disease.

As with most fertility treatments, success depends on many factors, including the woman’s age and whether a cause of infertility has been identified.

Sometimes no embryos are suitable for transfer to the womb, for reasons including:

• not enough eggs are produced or fertilised in the first place 
• removing the cells to be analysed damages the embryos 
•  all the embryos are affected by the genetic disease.

In 2006, for women receiving PGD, the percentage of cycles started that resulted in a live birth was:

• 33.3% (23/69) for aged women under 35 
• **(9/48) for women aged between 35-37 
• **(2/27) for women aged between 38-39 
• **(0/16) for women aged between 40-42 
• **(0/10) for women aged between 43-44 
• ** (0/1) 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).

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