Chromosome abnormalities in pregnancy

Reviewed by Dr Philip Owen, consultant obstetrician and gynaecologist



Chromosome abnormalities of the developing baby (foetus) are uncommon, but many parents are concerned their baby might be affected.

Several tests are now available to screen for and diagnose these abnormalities at an early stage of the pregnancy.

What are chromosomes?

Chromosomes are the thread-like structures present within the central part (nucleus) of living cells.

Chromosomes are composed of DNA (deoxyribonucleic acid) and therefore are the physical form in which our genes exist.

Humans have a total of 46 chromosomes. Two of these are the sex chromosomes, which are called X and Y.

The composition of the chromosomes for females is 46XX, while for males it is 46XY. Each half of the pair of chromosomes is inherited from each parent.

Chromosomes are like library shelves - each houses a large number of 'books', which are the individual genes.

What is a gene?

A gene is a defined piece of genetic information that contains the instructions necessary for:

a particular function of a cell or

cell processes to manufacture a particular protein.

Decoding DNA

The Human Genome Project has already decoded the overall sequence of human DNA contained within the chromosomes.

The DNA appears to provide the information (genetic code) for about 30,000 different proteins. Proteins are an essential part of the body's make-up.

These proteins can then undergo further chemical changes that may affect their function.

This means the total number of proteins that can be produced from the information in the chromosomes is in the order of hundreds of thousands.

Chromosome abnormalities

Abnormalities can involve the chromosomes themselves or can involve just one or more genes.

Most gene abnormalities do not result in an abnormality in the baby unless an abnormal gene is inherited from both the mother and the father.

Usually, if only one copy of a gene is defective, the 'good copy' from the other parent will take over.

Chromosome abnormalities are more common than is generally realised.

More than 50 per cent of miscarriages in the early stages of pregnancy are due to abnormalities of the chromosomes.

In about 0.5 to 1 per cent of all live births, the baby is found to have a recognisable chromosome abnormality.

Down's syndrome

The most well-known type of chromosome abnormality is Down's syndrome. Instead of having two no. 21 chromosomes, individuals affected by the condition have three.

Down's syndrome causes a low IQ and distinct features such as short limbs and a characteristic wrinkle around the eyes. Heart defects are also common and are present at birth.

It is not possible to predict whether a Down's syndrome baby will be mildly or severely mentally affected.

The age factor The risk of having a baby with Down's syndrome is:

1 in 1500 at age 20.

1 in 350 at age 35.

1 in 50 at age 43.

Other syndromes

Children with an extra chromosome no. 13 (Patau's syndrome) or no. 18 (Edward's syndrome) seldom survive beyond birth. These conditions are less common than Down's syndrome.

If an embryo has a 45X0 composition in the chromosomes, with the other sexual chromosome missing, it will be a girl with Turner's syndrome.

These girls are short and cannot have children because their ovaries are underdeveloped. Their mental development is normal, but certain heart defects are common.

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