An NT (Nuchal Translucency) scan must be performed between 11 weeks and 13 weeks plus 6 days of pregnancy.
This precise window is critical because it relies on the baby’s specific size to ensure accurate screening results.
Why the Timing is So Strict
- The Starting Boundary (11 Weeks): Before 11 weeks, the baby is too small for the sonographer to accurately measure the fluid-filled space at the back of the neck (the nuchal translucency).
- The Ending Boundary (14 Weeks): At 14 weeks and beyond, the baby’s developing lymphatic system absorbs this excess fluid, making the measurement clinically unreliable.
- The Size Metric: Medically, the baby’s Crown-Rump Length (CRL)—the measurement from the top of the head to the buttocks—must fall strictly between 45 mm and 84 mm for the scan to be accurate.