15/01/2010 12:22 GMT | Updated 22/05/2015 10:12 BST

D: What Is Down's Syndrome Testing?

Down's syndrome is a chromosomal abnormality that affects one in every 500 babies. Babies born with Down's syndrome carry an extra chromosome, which causes the condition. The syndrome causes both physical and learning disabilities.

Testing in pregnancy is available to determine the likelihood of Down's syndrome, and is particularly recommended for older mothers, or those believed to be at a higher genetic risk.

Testing is done via three different methods – a blood test along with ultrasound scanning (known as the triple test); an amniocentesis; or chorionic villus testing (CVS).

The triple test uses information from ultrasound scans, which includes looking at the length of the baby's spinal chord. This is done in conjunction with a blood test that examines the levels of certain chemicals in the mother's blood. From this, a risk factor for Down's syndrome is determined.

For a definitive and certain result, amniocentesis or chorionic villus sampling (CVS) is undertaken. Amniocentesis involves taking a small sample of amniotic fluid from the uterus, which is then tested in a laboratory for the presence of the extra chromosome.

Similarly, CVS involves taking a small sample of tissue from the placenta, which again will be analysed for the extra chromosome.

While both of these procedures generate an almost pinpoint result, they do also carry a small risk of miscarriage. One in 100 women will suffer a miscarriage following an amniocentesis; while two in every 100 women will miscarry after CVS.

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