At the main UK centre for pre-implantation genetic diagnosis, or PGD as it is known, the hundredth baby resulting from the use of this technique has just been born. Staff at the unit in Guy’s and St Thomas’ in London celebrated the event, and the relief that it has brought to families who otherwise would have faced the risk of having a baby affected by a severe genetic disease. But what exactly is PGD? How does it work, and what can it do?

Many of the serious diseases that affect us result from changes to our genes. Over 6000 different disorders, many of which are lethal or which result in severe, progressive disability arise from changes in single genes. Individually these are usually rare, but because there are so many of them genetic disease is not uncommon. Every year about 15,000 babies are born in Britain with a significant danger of disability or chronic ill health as a result of genetic changes. In recent years progress in genetic research has been rapid, and as a result sometimes it is now possible to test the DNA of someone known to be at risk of a given condition and see if they carry any changes to the gene responsible that will result in them developing a particular disease.

This progress has crested the opportunity for couples who are aware of the existence of a genetic threat to make choices about whether or not to have children, and what to do if and when the woman becomes pregnant. Prior to the development of PGD, the options for  couples’ at risk, if they wished to avoid the birth of a child with a severe genetic disease, were either not to become pregnant or to have a genetic test done on the developing fetus and if it was found to be affected, terminate the pregnancy. Both of these are difficult choices and carry significant physical and psychological consequences not only for the woman but also for her partner too.

Understandably, some couples prefer to remain childless rather than run the risk of finding that their developing foetus has a serious genetic disease and consider terminating a wanted pregnancy. The development of PGD means that, for couples for whom termination of an affected pregnancy is unacceptable there is the possibility of having a child that is unaffected by a life limiting genetic disease.

PGD works by combining the techniques of genetic testing with those of In-Vitro Fertilization (IVF). It is possible because a cell can be removed from a very early embryo (in the first couple of days following fertilisation) without harming any subsequent baby. It works like this. A woman who wishes to become pregnant using PGD is first given strong drugs to stimulate her ovaries to produce more eggs at the same time than the one or two that are usually released in her monthly cycle. There are collected and fertilised in the laboratory using her partner’s sperm. From the embryos that result a single cell is removed, and the DNA in it is analysed to see whether or not it carries the genetic changes that will result in the disease which the couple which to avoid. If an embryo does not carry the changes, and if it looks healthy, then it is put in to the woman’s womb where, hopefully it will implant, and develop into a baby that is free from the family’s genetic risk. Any spare unaffected embryos may be frozen for possible future use, whilst affected ones many be discarded or (if consent is given) used for research. Unaffected embryos many also be discarded or used for research once the couple decide their family is complete and with the proper consent.

Whilst it sounds simple, PGD is not necessarily straight forward in practice. The drugs used to stimulate the women’s ovaries are powerful and may have unpleasant side effects. Nor is there any guarantee that the embryos produced by the IVF process will be free of the genetic change that results in the disorder that the couple are trying to avoid; even if they are, putting the embryo in the womb many not result in a pregnancy. Nor does PGD guarantee of a healthy baby. Assuming the process works, and the woman gets pregnant all she will know is that her baby does not have the specific genetic condition that has been looked for. Other cases of ill health or disability in babies cannot be ruled out, but the elimination of at least one serious threat is a great relief. In future it many be possible to screen single cells taken from embryos for a wide range of genetic disorders. Today we can only look for conditions where the genetic changes are known and the couple are definitely at risk.

Is PGD the route to “designer babies” with innate abilities or talents derived from their parents? No! Positive attributes, such as intelligence, beauty, sporting ability or musical genius are controlled by many different genes and by their interaction with each other and with environmental and lifestyle factors. The genes associated with a particular characteristic are generally scattered at random on the 23 pairs of chromosomes that hold the genetic information in our cells. A very successful course of IVF is unlikely to produce more than ten or a dozen embryos, and many produce less than this. Some of these embryos are likely to be unsuitable for implanting anyway, so the chances of finding one which has the right combination from perhaps dozens or hundreds of different genes in any one of perhaps 6 to 8 embryos is mathematically remote – even assuming we know which genes to look for, and could quickly and reliably test them. Even if it were theoretically possible, anyone wanting to attempt this in the UK would have to apply to the Human Fertilisation and Embryology Authority (the body that controls PGD and IVF clinics) for permission. Whether they would be prepared to approve any such request is far from certain.

Not withstanding its limitations, PGD has given hope to couples facing the threat of genetic disease in their children that there is a chance of escape from their genetic threat or that they will be able to have a baby free of a specific genetic disease, and that they will not have to either remain childless or face the possibility of an ante-natal genetic diagnosis and the subsequent agonising decision about terminating the pregnancy. For coupes in this situation, the development of PGD, even with all its limitations, offers a humane technological response to what might otherwise be a human tragedy.

Alastair Kent
Director - Genetic Interest Group

The Genetic Interest Group (GIG) is a national alliance of patient organisations with a membership of over 130 charities which support children, families and individuals affected by genetic disorders.

Learn more about the Genetic Interest Group (GIG)