When is it done:
75% for Down's syndrome.
The mother has an ultrasound scan whereby the collection of fluid at the back of the baby’s neck is measured. A high reading may point to abnormalities such as Down’s syndrome or a heart defect. If this is the case, another diagnostic test such as amniocentesis or CVS (Chorionic Villus Sampling) is needed to provide a more conclusive answer regarding the chromosomes. Scans later in pregnancy need to be done to rule out cardiac problems.
The nuchal scan is the single most effective screening method for Down's syndrome provided the operator is properly trained and accredited by the Foetal Medicine Foundation (London, UK). Names of their accredited centres in South Africa can be found on their website.