High Quality Content by WIKIPEDIA articles! A nuchal scan is a sonographic prenatal screening scan (ultrasound) to help identify higher risks of chromosomal defects including Down`s syndrome in a fetus, particularly for older women who have higher risks of such pregnancies. High thickness measurements are also associated with congenital heart defect. The scan is carried out at 11–13.6 weeks pregnancy and assesses the amount of fluid behind the neck of the fetus - also known as the nuchal fold or the nuchal translucency. Fetuses at risk of Down`s Syndrome 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 fetal viability. Its high definition imaging may also detect other less common chromosomal abnormalities.