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Chromosome abnormalities in pregnancy
Reviewed by Dr Philip Owen, consultant obstetrician and gynaecologist
Molar pregnancy
A molar pregnancy is a rare form of pregnancy that has an abnormal composition of chromosomes and does not continue to develop normally.
Despite surgical removal of the abnormal tissue by D & C, 10 to 15 per cent of complete molar pregnancies continue to grow, as do about 1 per cent of partial molar pregnancies.
This is why after a molar pregancy a woman is registered for follow up at one of three centres in the UK. There, urine samples will be tested at regular intervals to ensure the pregnancy hormone hCG is disappearing.
This is because the abnormal tissue produces hCG, so its presence is an indicator of persistent or recurrent molar pregnancy. It is advisable not to become pregnant again until the all-clear is given.
Screening tests
If you wish to know whether your baby is affected by Down's syndrome before the birth, you will be offered one or more screening tests.
Screening tests indicate whether an abnormality is likely, without risk to the foetus.
If you are at high risk, you will be offered an amniocentesis test.
A high-risk result does not mean the baby is affected - it just means you may want to have a diagnostic test.
Nuchal translucency measurement
Down's babies have a bigger space than unaffected pregnancies.
By combining this measurement with your age and sometimes with hormone measurements in your blood, it is possible to calculate the risk of carrying an affected baby.
If the risk is high, you may want to consider a diagnostic test.
Diagnostic tests
There are two types of diagnostic test: amniocentesis and chorionic villus sampling (CVS). Which test you are offered will depend on the hospital where you plan to give birth.
Amniocentesis
Using ultrasound to guide the way, a fine needle is inserted through the mother's abdomen and into the fluid surrounding the foetus. 20ml of amniotic fluid is removed and sent for analysis.
Bleeding or leaking of fluid from the vagina is not normal.
You should contact the hospital if this happens.
Amniocentesis carries a 0.5 to 1 per cent risk of miscarriage within three weeks of the test.
CVS carries a 1 to 2 per cent risk of causing miscarriage.
Chorionic villus sampling (CVS)
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 laboratory.
The procedure usually takes 5 to 10 minutes. It is a little more uncomfortable than amniocentesis for the mother. The foetus is unaware of the procedure.
With modern techniques, a preliminary result may be available within 48 hours. The final result will be ready in two weeks.
Are these tests safe?
Having information about your baby's chromosomes has to be balanced against the small, but real, risk of losing the pregnancy.
If an abnormality is detected
The decision whether to continue with the pregnancy always rests with the parents, and some parents may decide to continue.
If an abnormal result would make no difference to your wish to continue with the pregnancy, you may decide against having diagnostic tests that carry a risk to your unborn baby.
Ultrasound is used to measure the fluid space at the back of the foetus' neck at 11 to 14 weeks. (Nucha, pronounced nooka, is the nape of the neck).
Analysing cells from the foetus is the only way a chromosome abnormality can be diagnosed with certainty. A diagnostic test is performed when:
Amniocentesis analyses a sample of the fluid surrounding the foetus in the womb (the amniotic fluid). It is performed from 15 weeks of pregnancy onwards.
CVS analyses a sample of the placenta and is performed from 11 weeks onwards.
Before going ahead with the tests, make sure you have considered the implications for you and your unborn baby.
There is no treatment for chromosome abnormalities. If a diagnosis is made before birth, you may be offered a termination of pregnancy depending on the nature of the abnormality.
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