Down's Syndrome is a chromosomal abnormality that affects 1:700 pregnancies in the UK and is the commonest cause of mental and physical handicap. Although the individual risk of Down's Syndrome increases with age, the majority of Down's Syndrome babies are born to younger women. Therefore, all women whatever their age, have a small risk of delivering a baby with Down's Syndrome.
The only way to know for sure whether or not the fetus has a chromosomal abnormality is by having an invasive test such as CVS or Amniocentesis, but these tests carry a risk of miscarriage of about 0.5%.
However, an early scan in association with a blood test from the mother known as the Combined Test, can give an accurate estimate of the risk of Down's Syndrome without a risk of miscarriage. The results of this test can help you decide whether or not to take the risks of a CVS or amniocentesis.
The Combined test involves a scan performed between 11 – 13 weeks and 6 days, which measures the amount of fluid behind the neck of the fetus (nuchal translucency) as well as an early detailed examination of the fetus.
A blood sample from the mother is taken to measure the levels of two hormones produced by the placenta (ß·hCG and PAPP·A). The results of the test are normally available on the following working day.