Amniocentesis and antenatal diagnosis

When a pregnant woman is known to have a chance of bearing a child with a genetic defect, it may be desirable to diagnose the condition in the fetus. This can be done by taking some cells from the fetus by drawing a few millilitersof amniotic fluid with the help of a hypodermic needle; The technique is called amniocentesis (Fig. 24.8) and is usually performed at 15th week of pregnancy, to allow enough time for safe abortion if recommended. The amniotic fluid has free cells of fetal origin, which can be cultured and tested in various ways e.g. karyotype, enzyme production and restriction site pattern analysis of its DNA. At least 35 diseases which can be identified by this technique are known. If disease is detected through such an antenatal diagnosis, abortion of fetus can then be recommended. However, if abortion is not acceptable to parents, there is no point in carrying out antenatal diagnosis.

It is possible to identify the disease now within 2 months of pregnancy, unlike an 18 week period required earlier. The number of disease specific DNA probes is also increasing at a fast rate, so that antenatal diagnosis by DNA analysis or linkage should be possible for all single gene defects. In recent years, the incidence of the disease thailassaemia in Cypriot community in Britain has fallen from 30 to 2 per year, due to the use of antenatal diagnosis. In U.S.A. on the other hand, there is a campaign against abortion and, therefore, also against antenatal diagnosis. In such cases there will be births of defective children and these may become patients for gene therapy discussed in the following section.