A Nuchal Translucency Scan is performed in the first trimester of pregnancy between 12 weeks 4 days and 13 weeks 4 days, although at Exact Radiology Clinics we recommend that this scan be performed at 13 weeks. The scan is used to determine if there is a higher risk of chromosomal anomalies, particularly Down Syndrome (Trisomy 21), but also two other significant chromosomal abnormalities called Trisomy 13 and 18.