Rare congenital and metabolic diseases

There are some conditions which may be inherited, and early testing may identify affected pregnancies. Some inherited conditions affect only one sex, so testing to identify the sex of the foetus may be offered.

The particular condition, and the need for testing in an individual or couple, is usually discussed when the medical and family history is discussed with the doctor at the first visit.

There are tests available which can check specifically for chromosomal abnormalities and spina bifida, as well as for some rarer conditions. The two most commonly offered are chorionic villus sampling and amniocentesis.

Chorionic villus sampling (CVS). This test is usually performed at between nine and twelve weeks gestation. It involves identifying the developing placenta with ultrasound, and taking a tiny sample of it to check the chromosomes. (The chorionic villi are embryonic tissue which form part of the placenta.)

The technique may be performed by inserting a very fine needle either through the woman’s abdominal wall (using local anaesthetic), or via a fine probe inserted into the vagina and through the cervix. The procedure is done by experienced doctors, under vision with ultrasound, and only takes a few minutes.

The results of this test, which gives information about chromosomal abnormalities, the sex of the foetus, but not about spina bifida, are usually available to the doctor who ordered the test within three weeks or so. Depending on the result of the test, a woman may decide to terminate the pregnancy, and this procedure would involve a routine abortion.

The risk involved with this particular test is a very small chance of miscarriage. There is always a ‘background’ risk of miscarriage in every early pregnancy (about 2 per cent at ten weeks). Having a CVS may increase this by another 1 per cent.

The test is offered to women whose chance of an affected pregnancy is the same, or greater, than the risk of miscarrying as a result of the test (for example women who will be over 37 years of age when the baby is due to be born).


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