Testing for Downs Syndrome

Testing for Downs Syndrome and Other Disorders

Even before conception many people worry about the risk of a baby being born with a certain abnormality. A couple may, for example, already have an abnormal baby, or a relative may have an abnormality. in these circumstances a genetic counsellor can often estimate the risk of a particular disorder occurring in a baby. The accuracy of a prediction can, however, only be made with a reasonable certainty if the precise nature of the disorder in question is known. The parents are usually seen in an out-patient clinic where a detailed family history is taken. This enables a family tree to be drawn up indicating any members of the family who may have suffered from the disorder.

Most abnormalities are not inherited in a simple dominant or recessive way. Indeed many of the fortunately rare congenital abnormalities result from a complex interplay of genetic and environmental factors. Examples include hare lip, cleft palate and congenital dislocation of the hip joint, all of which can be successfully treated in the first twelve months of life. in such cases, the counsellor can only give a very approximate risk of the abnormality occurring. However, even when there is a family history, the risk of it recurring is usually small.

Testing for Downs SyndromeGenetic counselling and the diagnosis if abnormality in the unborn baby early in pregnancy are complementary services. Sometimes parents, having received genetic counselling, decide to have a baby knowing that it will be possible to detect the abnormality and terminate the pregnancy should this prove necessary. Unfortunately, only a few abnormalities can bi diagnosed early in pregnancy. When this approach is feasible it does, at least, give the parents the chance of having a baby free from the defect in question.

The most common reason for genetic testing is when a mother is at risk of giving birth to a baby with spinabifida or downs syndrome. There are various types of spinabifida, but basically the abnormality means that some of the bones of the vertebral arches have failed to form their usual protective role of encasing the spinal cord. in this rare instance, a segment of spinal cord, usually in the lower part of the back, becomes exposed on the surface of the skin. When this happens, a chemical, known as alpha fetoprotein, is present in large quantities in the waters (amniotic fluid) that surround the unborn baby.

A sample of the fluid can be removed at about the sixteenth week of pregnancy and analysed. Although a very high level of alpha fetoprotein suggests that spinabifida is present. the level does not always rise in some of the milder forms of the condition. Cells from the skin of an unborn baby are continually shed into the fluid. If a specimen of this fluid is removed by amniocentesis the cells can be separated and grown in a special broth. After two weeks, if the cells are looked at with a powerful microscope, the chromosomes in each cell can be seen and counted.

It is possible to diagnose downs syndrome in this way. This test is generally performed for women who have somewhat greater risk of giving birth to a downs syndrome baby, namely those over thirty-five years, and those who have already given birth to a downs syndrome baby. The results of chemical tests done on cells that an unborn baby has shed into the amniotic fluid can reveal certain rare metabolic disorders.

These tests are only applicable to women who have already given birth to a baby with a rare metabolic disorder, because both parents carry an abnormal recessive gene. If a mother is a carrier of the rare disease of haemophilia there is a one in two chance that male babies born to her would be affected. Chromosome analysis of the unborn baby’s skin cells enables the sex of the baby to be determined early in pregnancy. A mother may feel that if she is carrying a boy in these circumstances she is justified in having the pregnancy terminated.

Quite commonly a baby’s facial features and even the colour of his hair or eyes alter with growth; a resemblance between him and other members of the family may only develop later. A mother might wonder how tall her baby will be. Height is one genetically determined characteristic that is modified by the environment in childhood. Over-eating will not make a baby taller. However, if a child is seriously undernourished, he may not achieve his genetically intended height. A child’s personality is also influenced by the environment in the early years. The laughter that lights up a child’s face generally reflects a happy home environment.