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Referral & Treatments
Pre-implantation Genetic Diagnosis
- Prenatal diagnosis
- Pre-implantation Diagnosis
Sometimes babies and children die because of inherited genetic disease. Some of these diseases can be detected before the baby is born, by pre-implantation or by prenatal diagnosis.
Prenatal Diagnosis
In prenatal diagnosis, cells are removed at 12-16 weeks of pregnancy. The techniques used are known as chorionic villus sampling (CVS), where part of the placenta is removed, or amniocentesis, where some of the cells floating around the fetus are removed. The advantage of these tests is that they are very accurate. The disadvantages are that there is a small risk of damaging the embryo or causing a miscarriage.
The amniocentesis test can take up to three weeks for the result to be known. If the fetus is affected, then the parents may be faced with a decision to terminate the pregnancy at a relatively late stage (16-20 weeks). Unfortunately in some patients each subsequent pregnancy may have affected fetuses and this can lead to repeated pregnancy terminations.
Pre-implantation Diagnosis
Pre-implantation diagnosis is the diagnosis of genetic disease before the pregnancy has started by testing the embryos before they are put into the uterus (womb). One or two cells are removed from the embryo and these are tested. If the embryo is judged to be free of the disease it can be put back into the uterus and hopefully develop into an unaffected baby.
The advantages of this test are that it is performed much earlier than the prenatal diagnostic tests, and as only those embryos free of disease will be transferred into the uterus there is only a very small possibility that an affected child will be born. The disadvantages of the procedure are that the woman must undergo in-vitro fertilisation so that there are sufficient embryos to examine. In-vitro fertilisation does not give a guarantee of a pregnancy, only approximately 1:4 treatments result in a live birth.
World-wide there have been more than 150 normal children born after pre-implantation diagnosis. Our most recent results show that in good quality embryos, cells can be removed safely from 90% of embryos. We can test 90% of these cells and so far the tests have been correct in all cases for cystic fibrosis and also in the detection of male embryos. However, pre-implantation diagnosis is a relatively new technique and should still be considered experimental. This is why we recommend that patients who do conceive have one of the two prenatal tests to confirm the correct diagnosis.
What is pre-implantation diagnosis used for?
Pre-implantation diagnosis has been used for two main groups of diseases, sex-linked disease, for example haemophilia and muscular dystrophy, and single-gene diseases, for example cystic fibrosis.
- For sex-linked diseases where males will be effected, the embryos are tested and only female embryos are transferred into the uterus. This is the only legal indication for embryo sexing. Embryo sexing cannot be performed for social reasons, such as a couple preferring to have a male or female child.
- For single-gene defects, embryos will be tested and unaffected embryos will be transferred into the uterus. In some cases there may not be any unaffected embryos, and we may discuss with you the possibility of putting unaffected but carrier embryos into the uterus. These carrier embryos do not have the disease but may pass it on to their children.
What is involved in pre-implantation diagnosis?
For the patient the procedures are the same as in normal IVF treatment, except that the embryo transfer will be performed on the third or fourth day after the egg collection. On the morning of the third day the embryo should have reached a stage in development where there are six to eight cells. Up to two cells are removed from the embryos and then the embryos put back into the culture medium. This procedure does not affect the chances of the embryo developing normally and forming a healthy baby. The cells are then tested using a procedure called polymerase chain reaction (PCR) which multiples signals from the cell allowing us to see if the cell I male or female or has a single-gene defect. When we have the results of all the tests, we will look at the embryos again and select the best embryos for transfer. We will normally transfer the best 2 embryos available that are apparently free from disease. A pregnancy test will be performed eleven days later.
- If you are interested in learning more about PGD contact Karen.M.Cooper@leedsth.nhs.uk