Genetic screening: amniocentesis and chorionic villus sampling

Because of the high risk of Down's syndrome among the babies of older mothers, in the UK mothers over the age of 35 years are usually offered free genetic screening by the National Health Service. Genetic screening refers to procedures used to examine an individual for the presence of a genetic disease or disorder. The most widely available genetic screening procedure for Down's syndrome is amniocentesis.

Amniocentesis is usually carried out at 15-16 weeks of pregnancy. It involves passing a very fine needle into the uterus, observed with an ultrasound image, and withdrawing a sample of amniotic fluid containing fetal cells. The karyotype of the fetal cells is then analysed to test for Down's syndrome. The fetal cells can also be cultured in a suitable medium in a laboratory so that further tests, such as DNA analysis, can be carried out.

Amniocentesis is performed under local anaesthetic and most women do not find it too uncomfortable. However, there is a 0.5-1 per cent risk of spontaneous miscarriage after the procedure. Therefore, amniocentesis is usually recommended only for those at high risk of carrying a Down's baby-In the 1970s, chorionic villus sampling (CVS)was developed in China. In CVS, a sample of cells is taken from the chorionic villus (small finger-like processes which grow from the embryo into the mothers uterus). The sample is obtained either by inserting a needle through the abdomen, or inserting a catheter. The fetal cells in the sample can then be analysed in the same way as for amniocentesis.

CVS can be carried out between week 8 and week 12 of pregnancy. If the test shows the fetus has Down's syndrome, a decision about abortion can be made earlier than with amniocentesis. Early abortions are usually less difficult, both physically and mentally, than later abortions. However, a higher risk of miscarriage is associated with CVS than with amniocentesis.

Until recently, a mother's age was the only factor available to assess the risk for Down's syndrome. Now biochemical markers are being discovered for the condition. For example, women with a high risk of Down's syndrome pregnancies tend to have about twice as much chorionic gonadotrophin (a sex hormone produced in placenta cells) in their blood serum as women with normal pregnancies. Tests for these biochemical markers cannot show the presence of a Down's baby, but they can be used in conjunction with the mother's age to predict the probable risk of having a baby with Down's syndrome. If the risk is high, the mother can then decide whether to have an amniocentesis or CVS.