What is Downs Syndrome?

Dr Maureen Norrie has written this piece on a topic that undoubtedly concerns almost all expectant and considering parents. Down’s Syndrome.
Here are the facts and background to this syndrome which affects around 1 in every 800 children born.

Down’s Syndrome is perhaps the commonest genetic abnormality in this country, characterised by a particular physical appearance and limited intelligence. It is caused by abnormalities in the baby’s genes, and as such has been present since the moment of conception.

That is: it is not due to anything which may, or may not, have happened in pregnancy or labour – and could not have been prevented by any measures undertaken by the mother in pregnancy.

The term “syndrome” means a group of symptoms and signs of “disordered function” which are physically related in some way. By itself, the concept of a “syndrome” does not include a precise cause of an illness or condition. It does provide a basis for investigating a condition, so that a cause may later be found.

Children with Down’s syndrome (once called Mongolism) have been born for at least hundreds – most likely thousands – of years. In 1866, John Langdon Down wrote down a description of the physical and intellectual features associated with this condition, and the condition came to bear his name. However, it was almost a century later that the cause of this condition became known.

In 1959, it was discovered that the cause of Down’s syndrome was an abnormality of one of the pairs of chromosomes: chromosome 21. In children with Down’s syndrome, there is actually an extra chromosome 21 (three instead of two); a newer name for Down’s syndrome – trisomy 21 – reflects this. (The person who discovered this lived until fairly recently).

At present, a overall total of around 1 in 800 children are born with Down’s syndrome. Another way of considering this is that around 799 out of 800 are not.

There is a well-recognised association between the age of the mother and the risk of giving birth to a child with Down’s syndrome: broadly speaking, the older the mother, the higher the risk.

In a pregnant woman of 35 years old, there is about a 1 in 365 chance that the baby will have Down’s syndrome. At the age of 40 years, the risk has risen to about 1 in 100; and by the age of 45 the risk is higher still, about 1 in 50. For these reasons, it is usual to offer women over about 35 the opportunity to “screen” the pregnancy for the possibility of this condition.

For men, the situation is rather less certain. Most people agree that there does not appear to be an increased risk of fathering a baby with Down’s syndrome until after age 55 years. After this age – some researchers have found an increased risk in fathering a baby with Down’s syndrome after this age; others have not. In other words, the jury is still out.

A generation ago, most babies (over 50%) with Down’s syndrome were born to older mothers. At present, the percentage of all babies with Down’s syndrome born to women over 35 years is around 20% (although women in this age group account for only around 5% of pregnancies).

This is partly because the actual birth rate in older women has plummeted within the past 20- 30 years, for a combination of reasons including a move away from large families (where some women may have become pregnant almost every other year), and the introduction of effective contraception (including sterilisation – still mainly sought by those who feel that their family is now complete). It may also be in part because of pre-natal screening in this age group.

However, young women can (and do) give birth to babies with this condition. At the present time, around 80% of Down’s children are actually born to younger mothers – under 35 years. Although the risk to an individual is small, about 95% of all babies are born to mothers in this age group.

In younger women (under 35 years) the overall risk of having a Down’s syndrome baby is around 1 in 1000; for the individual this risk is much less than that of her older cousin. However, although preliminary pre-natal tests for Down’s syndrome involve only a blood test, they are not yet foolproof. They do identify most (around 60% of) babies with Down’s syndrome (which means that they miss some); and they also have a “false positive” rate of around 5%-7% (which means that around 1 in 20 “positives” are wrong).

These results can, however, prompt further, more detailed, tests (tests which are not offered routinely to everyone because they have their own problems, including a slight miscarriage risk). If the preliminary tests appear positive, the detailed tests can then be offered to the mother – who makes the final decision on how far to continue with these tests.

Article by Dr Maureen Norrie

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