Pre-implantation genetic diagnosis (PGD)

Pre-implantation genetic diagnosis is used to try to prevent embryos that are produced by in vitro fertilisation having a genetic disease that has been passed on to them by one of their parents. Genetic disorders such as cystic fibrosis can produce severe symptoms and can shorten life. Pre-implantation genetic diagnosis allows parents having IVF to avoid this.

Our article on pre-implantation genetic screening outlines the techniques used to check the genetic make-up of the embryos produced by IVF. Here we look at why PGD is used, what conditions it can be used to detect, and how regulations vary between different countries.

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Why might pre-implantation genetic diagnosis be recommended?

Some people have had a genetic test that shows they carry a gene associated with a genetic disease that can be passed on to their children, and they request pre-implantation diagnosis to avoid that. PGD is also advised if:

• You have had several pregnancy terminations because the baby had a serious and life-threatening genetic condition.
• A known serious genetic disease runs in your family: you may have a brother or a sister with cystic fibrosis, for example.
• You have given birth to a child who has been diagnosed with a known genetic disease.

Which countries allow pre-implantation genetic diagnosis?

In many countries, pre-implantation genetic diagnosis is not allowed at all; the Catholic Church does not accept it, and many other religious groups and national governments are also against it. PGD is not allowed in Chile, Switzerland, China, the Ivory Coast, the Philippines, Algeria, Ireland or Austria. For many years Germany ruled that pre-implantation genetic diagnosis was illegal but, in 2011, changes to legislation were made to allow it in some circumstances.

Sometimes, regulations vary within countries; in the USA, for example, the rules on pre-implantation genetic diagnosis vary between states. Similarly in Europe, member states can make their own decisions on whether it is possible or how it can be applied.

In countries where pre-implantation genetic diagnosis is permitted, for example Spain, France, Sweden, the UK, the USA, Norway and Denmark, it is usually tightly controlled. In the UK, the Human Fertilisation and Embryology Authority (HFEA) allows only some diseases to be screened for. It also has strict criteria that newly proposed diseases must comply with, and insists that testing is done by a fertility clinic properly licensed to carry out pre-implantation genetic diagnosis.

Pre-implantation genetic diagnosis and sex determination

It is possible that pre-implantation genetic diagnosis can be performed to choose an embryo for transfer on the basis of its sex. Sex selection is not considered ethical in many countries; it is banned in Canada, Taiwan and the UK, but it is allowed, if not routinely practised, elsewhere. There is widespread debate internationally about whether sex selection for family balancing reasons, rather than medical reasons, is ethical.

However, when the female partner of a couple having IVF is a carrier of a genetic disease that is carried on the X-chromosome, the easiest and safest way to select embryos that will be free of the disease is to select ones that are female and not male. This does not, however, prevent the X-linked gene being passed on and the child, although it will not have the genetic disease, could still be a carrier.

Pre-implantation genetic diagnosis: which diseases does it test for?

Tests are now possible for a range of genetic diseases that are due to a fault in a single gene, including those that are linked with the X-chromosome. These include:

• Cystic fibrosis.
• Huntington’s chorea.
• Duchenne muscular dystrophy.
• Fragile X syndrome.
• Haemophilia.
• Myotonic dystrophy.
• Beta-thalassaemia.
• Sickle cell anaemia.
• Spinal muscular dystrophy.

All of these diseases cause premature death or considerable disability or illness during life and are considered to have serious consequences. Some fertility clinics around the world test for less serious genetic conditions or less well-proven mutations, but you need to check this with the individual institution and make sure this fits in with national regulations.

Currently, there is no agreement about whether genes such as BRCA1, that pre-dispose to breast cancer, should be excluded using pre-implantation genetic diagnosis, as not everyone with that mutation goes on to develop a breast tumour.

Pre-implantation genetic diagnosis to select a saviour sibling

A more controversial use of pre-implantation genetic diagnosis is for a couple to have IVF to select an embryo that carries similar HLA tissue typing to a child already born, where this child has a life-threatening non-genetic disease such as childhood leukaemia. This can be successfully treated using a stem cell donor transplant from a closely matched donor. If such a donor cannot be identified, having another child who can act as a donor is an extreme step that some parents are driven to take.