Individuals with Down syndrome have distinct physical characteristics; generally they are more similar to the average person in the community than they are different. The physical features are important to the physician in making the clinical diagnosis, but no emphasis should be put on those characteristics otherwise. Not every child with Down syndrome has all the characteristics; some may only have a few, an others may show most of the signs of Down syndrome.

Over a 120 features have been described in Down syndrome. Many children with the syndrome have no more than six or seven of these. Some of the features are those that are either particularly useful in recognizing the condition, or have some relevance to parents.

Diagnostic tests are amniocentesis and chorionic villus sampling (CVS). Amniocentesis is the commonest form of antenatal diagnosis for Down syndrome. With amniocentesis a needle is passed through the mother’s belly into the womb to sample fetal cells in the amniotic fluid. These cells are then sent for chromosome analysis. The result of the chromosome test becomes available about 2 weeks after amniocentesis. An ultrasound is used to safely pass the needle. The test is usually done between 14 and 18 weeks of pregnancy and is fairly safe, but there is a small risk of miscarriage. If it does happen after amniocentesis, it usually occurs two to three weeks after the event. The risk of miscarriage occurring after amniocentesis is about 1 in a 100. CVS samples cells from the chorionic villi, a structure in the womb that has fetal cells but is not the fetus. It is done between 9 and 12 weeks. Chorionic villi are made up of special rapidly dividing cells, which grow to become the placenta. Growth of these cells in the laboratory is therefore extremely quick, and it is possible for the chromosome result to be available a day or two after the procedure. It also has similar risks.

There are two types of tests available to pregnant women to test for Down syndrome. Diagnostic tests sample fetal cells and give a definitive diagnosis. Screening tests are relatively simple tests that find most of the fetuses with Down syndrome, but they also find some without. Screening tests need to be confirmed by a diagnostic test.

RELATIONSHIP OF DOWN SYNDROME INCIDENCE TO MOTHERS' AGE

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