A nuchal scan is a sonographic prenatal screening scan (ultrasound) to help identify higher chances for chromosomal conditions including Down 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 soft-tissue thickness within the nape of the fetal neck. There are two distinct measurements -- the nuchal translucency, which is measured earlier in pregnancy at the end of the first trimester, and for which there is a lower threshold for increased diameter, and the nuchal fold, which is measured towards the end of the second trimester. Fetuses with Down syndrome tend to have an increased 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.