Chromosome abnormalities of the developing baby (foetus) are uncommon but many parents are concerned that their baby might be affected. Several tests are now available to screen for and diagnose these abnormalities at an early stage of the pregnancy.

Chromosomes and genes
Human beings 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.

Chromosomes contain all our inherited genetic information (genes) and are numbered as pairs from 1 to 22, plus the two sex chromosomes. Each person has many thousands of genes, half of which are inherited from their mother and half from their father.

Abnormalities can involve the chromosomes themselves or can involve just one or more genes. Most gene abnormalities do not express themselves as abnormalities in the baby unless an abnormal gene is inherited from both the mother and the father.

Chromosome abnormalities
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.

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.

Screening for Down's syndrome in pregnancy
Down's syndrome is not common, but the chances of having an affected baby increase as the mother gets older. For example, the risk of having a baby with Down's at age 20 is 1 in 1500, at age 35 it is 1 in 350 and at age 40 it is 1 in 100. If a woman has had a previous pregnancy affected by Down's syndrome, then her risk is increased threefold.

If a close family member is affected, she may wish to be tested to see whether she herself has inherited a chromosome abnormality which increases the risk of having an affected child (balanced translocation).

Many women wish to know before the birth if their baby is affected and they are offered one or more screening tests.

Serum screening
This is a blood test performed at 15 to 20 weeks. It measures the levels of pregnancy hormones in the mother's blood. By combining these results with the mother's age, then it is possible to calculate the risk of the baby having Down's syndrome. If the woman is considered low risk after the blood test, then nothing further is usually necessary. If the result suggests she is at high risk of having an affected baby, she will be offered an amniocentesis test to see whether the baby is affected or not.

The blood test is not foolproof and, occasionally, a Down's baby will be born to a mother with a low-risk result. A high-risk result does not mean that the baby is affected, it just means that the woman may wish to consider a diagnostic test.

Nuchal translucency measurement
Using ultrasound at 11 to 14 weeks it is possible to measure the fluid space at the back of the foetus' neck. Down's babies have a bigger space than unaffected pregnancies. By combining this measurement with the mother's age and sometimes with hormone measurements in the mother's blood, it is possible to calculate the risk of carrying an affected baby. Again, if the risk is high then the woman may consider a diagnostic test.

Diagnostic tests
A chromosome abnormality can only be made with certainty by analysing cells from the fetus in the genetics laboratory. A diagnostic test is performed when:

a woman is considered at high risk because of the screening test.

a woman is considered at high risk because she has previously had a pregnancy affected by a chromosomal or genetic disorder.

an ultrasound examination has detected features or abnormalities indicating an increased risk of a chromosome abnormality.

a woman requests it because she is concerned that her baby has a chromosome abnormality.

There are two types of diagnostic test - amniocentesis and chorion villous biopsy (CVS). Which of these tests a woman is offered will vary according to the hospital in which she plans to give birth. It is important that the mother understands the implications and the purpose of these tests.

Are amniocentesis and CVS tests safe for the mother and baby?
Both tests carry a slight risk of causing a miscarriage. Before deciding upon screening and diagnostic tests, it is, therefore, important for the expectant mother to discuss any concerns she may have with her obstetrician. Having information about the baby's chromosomes has to be balanced against the small but real risk of losing the pregnancy.

What happens if a diagnostic test shows abnormalities?
There is no treatment for chromosome abnormalities. If a diagnosis is made before birth, depending on the nature of the abnormality that has been detected, the woman may be offered a termination of pregnancy. The decision whether or not to continue with the pregnancy always rests with the parents and some parents may decide to continue. Before any diagnostic test is carried out, it is important for the expectant parents to consider what difference an abnormal result would make to their wish to continue with the pregnancy.

How are the diagnostic tests carried out?

Amniocentesis Amniocentesis analyses a sample of the amniotic fluid. It is performed from 15 weeks gestation onwards.

Using ultrasound to guide the way, a fine needle is inserted through the mother's abdomen and into the fluid surrounding the fetus. Then, 20mls of amniotic fluid is removed and sent to the laboratory for analysis. The procedure usually takes 5 to 10 minutes and the mother will feel only slight discomfort. The foetus is unaware of the procedure.

After the procedure, the mother should rest for the day. She may feel slight cramping. Bleeding and leaking of fluid from the vagina is not normal and she should contact the hospital if this happens.

A preliminary result may be available within 48 hours with modern techniques with the final result ready in two to three weeks. Amniocentesis is not risk free and carries a slight risk (0.5 to 1 per cent) of causing a miscarriage. Between 1 in every 100 and 1 in every 200 amniocentesis procedures will cause a miscarriage, usually within three weeks of the procedure.

Chorion villous sampling (CVS)
CVS analyses a sample of the placenta and is performed from 11 weeks onwards. Using ultrasound to guide the way, a needle is inserted through the mother's abdomen into the developing placenta. Suction is applied and a small sample of tissue is sent to the genetics laboratory. The procedure usually takes 5 to 10 minutes and the procedure is a little more uncomfortable than amniocentesis for the mother. The foetus is unaware of the procedure.

After the procedure, the mother should rest for the day. She may feel slight cramping. Bleeding and leaking of fluid from the vagina is not normal and she should contact the hospital if this happens. A preliminary result may be available within 48 hours with modern techniques and the final result will be ready in two weeks.

CVS is not risk free and carries a 1 to 2 per cent risk is of causing a miscarriage. Between 1 in every 50 and 1 in every 100 CVS procedures will cause a miscarriage.

Miscarriages
Pregnancies with abnormalities of the chromosomes often end in miscarriage. More than 50 per cent of miscarriages in the early stages of pregnancy are due to abnormalities of the chromosomes.

A rare form of pregnancy called a molar pregnancy has an abnormal composition of the chromosomes and does not continue to develop normally. A complete molar pregnancy develops a placenta looking like 'a bunch of grapes' without an accompanying foetus.

Partial molar pregnancies develop an embryo that does not continue. Despite a D&C procedure, 10 to 15 per cent of the complete and about 1 per cent of the partial molar pregnancies continue to grow.

This is why, after a molar pregnancy, a woman is registered for follow up at one of three centres in the UK. This will involve providing urine samples at regular intervals to ensure that the pregnancy hormone hCG is disappearing. It is advisable not to become pregnant again until the all clear is given.

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