Prenatal diagnosis of Down’s Syndrome

Out of every 700 babies born, one will have the condition known as Down's syndrome. This chromosomal disorder gives the affected individual specific recognisable physical characteristics. However, more significantly, the person will have considerable learning difficulties with an average mental age of five or six years old by the time he or she reaches adulthood.

A Down's syndrome baby is prone to other major abnormalities of such vital organs as the heart, lungs or gut. It is therefore important that, whatever the opinion of the parents about the merits and drawbacks of prenatal screening for such conditions, they know what the condition is all about. It is only then, from an informed position, that they can rationally decide where they stand.

There are, of course, other (less common) chromosomal abnormalities apart from Down's syndrome, which the screening tests can detect. Some of them are fatal and the baby stands no chance of survival outside the womb.

The screening tests commonly used are what are known as the "triple test" and the "double test". They are similar, with equal efficacy in detection. Other names for the ‘Triple Test’ are the ‘Kettering’s Test’ and ‘Bart’s Test’. These names are rarely used.

Sensitivity of Triple Test

Significantly, the sensitivity of the ‘Triple Test’ is only about 60 - 75%. This means; with this blood screening test alone, potentially up to four out of every ten Down's syndrome babies will not be detected.

It is also true that there is a significant incidence of false alarms from the test. However, currently, it is the best widely available non-invasive screening test. Where the risk is considered high, the test is normally combined with detailed ultrasound scanning to look for the tell-tale features of Down's.

However, the only definitive tests, which establish the fact beyond any doubt, are the invasive ones where the fetal cells are analyzed and the chromosome make-up established.

An established diagnosis moves the issue to a new level of what to do. Many parents who wish to have the screening test and, if the need arises, the definitive diagnostic test, do so because they may want termination of pregnancy if the diagnosis is confirmed. A smaller percentage do so simply because they want to know and be prepared. The third group comprises those who decline the screening test altogether because it wouldn't make a difference one way or another.

Here, we do not pretend to analyze or even give an opinion on these different stances. Those profound questions surrounding the issue of what to do are well beyond the scope of this text. What we have set out to do is to give the facts on the subject of Down's and other chromosomal disorders and clarify them.

Continues next page