The chance that the fetus has Down's syndrome at 12 weeks is
higher than at birth because some fetuses with this condition die during pregnancy.
In 95 per cent of fetuses the nuchal translucency is low and the
chance that the fetus has a chromosomal abnormality is decreased. In five per cent of
pregnancies the nuchal translucency is high. In these cases the chance the fetus has a
chromosomal or other abnormality is higher than average but it is important to realise
that a majority of fetuses will be normal.
The only way to know for sure whether or not the fetus has a
chromosomal abnormality is by having an invasive test such as chorion villus sampling
(CVS) or amniocentesis. However these tests carry a risk of miscarriage of about one per
After the scan the estimated chance for Down's syndrome will be
discussed with you. It is up to you and your partner to decide whether or not the chance
of the fetus having a chromosomal abnormality is high enough to warrant having an invasive
test such as CVS.
Irrespective of whether or not you decide to have an invasive
test, it is recommended that you have a scan at 20 weeks to check for physical
abnormalities. This is particularly important in fetuses with a high nuchal translucency
because these fetuses have an increased chance of cardiac and other abnormalities.