A nuchal scan is a sonographic prenatal screening scan (ultrasoun) to help identify higher risks of Down syndrome in developing babies, particularly for older mothers who have higher risks of such pregnancies. The scan is carried out at 11-13 weeks pregnancy and assesses the amount of fluid behind the neck of the fetus - also known as 'the nuchal translucency'. Babies at risk of Down tend to have a higher amount of fluid around the neck. The scan may also help confirm both the accuracy of the pregnancy dates and the foetal viability. Its high definition imaging may also detect other less common chromosomal abnormalities.