What is nuchal translucency? This is the fluid that builds up under the skin at the back of your baby’s neck between 10 and 14 weeks of pregnancy and can be measured using ultrasound. All babies have some fluid, however, most babies with Down syndrome have a thicker nuchal translucency (NT). A posterior nuchal translucency scan is one way of assessing whether your baby may have Down’s syndrome; it is a screening test. Unlike screening, which only assesses your baby’s risk of having Down syndrome, diagnostic tests like chorionic villus biopsy or amniocentesis will give you a definitive diagnosis (although they also carry a small risk of miscarriage). While a posterior nuchal translucency band scan can’t tell you exactly if your baby is sick, it can help you decide if you need diagnostic testing. When is a nuchal translucency scan done? Posterior nuchal translucency scans are usually performed between 11 and 13 weeks + 6 days of pregnancy. scanning before 11 weeks is technically difficult because the baby is too small, and after 14 weeks too much fluid may be absorbed by the baby’s developing lymphatic system.