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Diagnostic tests

If the result of an antenatal screening is positive for the risk of Down’s syndrome or another abnormality, you will be offered one of the following diagnostic tests.

Amniocentesis

What is amniocentesis?

An amniocentesis is a simple and widely used procedure. It is performed from about 15 weeks of pregnancy and involves collecting a small sample of amniotic fluid from around the baby by inserting a needle through the abdominal wall. This fluid contains cells from the baby.

What will the amniocentesis tell me?

Down's syndrome is diagnosed using a technique called quantitative fluorescence polymerase chain reaction (QF-PCR). This provides a rapid diagnosis for Down’s syndrome, usually within 48 hours of the amniocentesis being performed. It also detects trisomy 18 and 13 and sometimes sex chromosome abnormalities. In some cases an examination of all chromosomes is carried out. The results of this can take up to two weeks.

No test can guarantee that your baby will be free of all birth defects, but if the result of the amniocentesis is negative it will almost certainly rule out Down’s syndrome or another major chromosome abnormality.

Is it safe to have an amniocentesis?

Amniocentesis is a procedure that has been offered for many years and over 25,000 are carried out in Britain each year. Its safety has been carefully studied and it is estimated that about 1 in 100 women who have an amniocentesis will miscarry as a result of the procedure.

Chorionic Villus Sampling (CVS)

What is CVS?

CVS is a diagnostic test for Down’s syndrome . This test is offered early in pregnancy in some hospitals (about 11 weeks) but may also be offered as an alternative to amniocentesis . CVS involves taking a sample of placental tissue by inserting a needle through the abdominal wall, or in some instances through the cervix.

What will the CVS tell me?

Down’s syndrome is diagnosed using a technique called quantitative fluorescence polymerase chain reaction (QF-PCR). This provides a rapid diagnosis of Down’s syndrome, usually within 48 hours of the CVS being performed. It also detects trisomy 18, 13 and sometimes sex chromosome abnormalities. In some cases an examination of all chromosomes is carried out. The results of this can take up to two weeks.

No test can guarantee that your baby will be free of all birth defects, but if the result of the CVS is negative it will almost certainly rule out Down’s syndrome or another major chromosome abnormality.

There is a small chance (about 1 in 100) that the CVS will not provide a conclusive diagnosis. If this happens you will need to have an amniocentesis.

Is it safe to have a CVS?

The risk of miscarriage is thought to be the same as for amniocentesis.

Detailed ultrasound scan

Anencephaly and Spina Bifida

A detailed scan is used to detect open neural tube defects such as anencephaly and open neural tube defect.

Down’s syndrome

It is not possible to make a diagnosis of Down's syndrome from an ultrasound scan. There are however certain physical features which may be associated with Down’s syndrome and can be seen on an ultrasound scan between 18 and 20 weeks. If any of these features are seen this would be a further indication for a diagnostic test but the absence of these features could not rule out Down’s syndrome.

What happens if my baby does have Down’s syndrome or an open neural tube defect?

Remember that the majority of diagnostic results will show that the baby does not have Down’s syndrome or an open neural tube defect. If the baby did have one of these abnormalities, you would have the chance to discuss how your baby could be affected. If you decided to have a termination of pregnancy your doctor or midwife would be able to make the arrangements for this. If you decided to continue with the pregnancy you could talk to someone about the special help and support you would receive to help you look after your baby.

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