Chromosomes are the carriers of hereditary information - the DNA. A normal cell contains 46 chromosomes (23 pairs). One half comes from the mother’s egg, the other half from the father’s sperm. After the fusion of egg and sperm (fertilisation), the form or number of chromosomes may be abnormal. This can bring about abnormalities in the fertilised egg.

All pregnancies, irrespective of age, have an approximate 3% risk of some form of chromosomal abnormality. An abnormality in the number of chromosomes occurs more frequently, however, as age increases.

In some instances, a fertilised egg that has chromosomal abnormality grows no further and fails. The woman may be completely unaware that fertilisation has even occurred, given that menstruation – at around the normal time - usually follows. However if growth stops at a slightly later stage, then the pregnancy may be detected and a miscarriage will follow.

According to estimates, half of all miscarriages in the first trimester are the result of chromosomal abnormalities. Chromosomal abnormalities can also lead to intrauterine foetal death. Sometimes a full-term child may be born with an abnormal number of chromosomes. This can result in the baby having a wide variety of health defects and/or retarded mental development or other problems.

An abnormality in the number of chromosomes - either more or less chromosomes - is the most common abnormality. One example of this is trisomy, in which one cell contains an extra chromosome. The best-known trisomy is Down’s Syndrome. In this case the 21st pair of chromosomes has an extra chromosome (trisomy 21), as a result of which there are now 47 chromosomes instead of 46 chromosomes. A trisomy can also occur sometimes on chromosomes 13 and 18. These abnormalities often lead to severe inborn abnormalities and death often even before, or shortly after birth. The severity of the condition and the chance of miscarriage are determined by the chromosomal abnormality.

Because many pregnancies end prematurely, it is not known precisely how often chromosomal abnormalities occur in pregnancy. However, data are known about the number of children born with chromosomal abnormalities such as Down’s Syndrome. It is clear that the chance of this syndrome increases as the age of the mother increases:

• Women of 20 have a 1 in 1528 chance.
• Women of 25 have a 1 in 1351 chance.
• Women of 30 have a 1 in 909 chance.
• Women of 35 have a 1 in 384 chance.
• Women of 40 have a 1 in 112 chance.
• Women of 45 have a 1 in 28 chance.

Just like Down’s Syndrome, other chromosomal abnormalities can occur at any age, but the older a woman is, the more frequently they occur.

As a result, pregnant women aged 36 or older are should be offered prenatal diagnosis. From that age the chance of discovering problems outweighs the possible risks of the procedures involved. These can include amniocentesis and chorionic villus sampling.