Must the NT scan be done at exactly 12 weeks?

Nuchal translucency (NT) screening is generally not required to be performed exactly at 12 weeks.

NT screening, also known as nuchal translucency measurement, is primarily conducted via ultrasound to assess the accumulation and thickness of fluid under the skin at the back of the fetal neck. During early pregnancy, the fetal nuchal translucency appears as an anechoic (fluid-filled) space that can be measured by ultrasound. The NT scan is typically performed between 11 and 14 weeks of gestation. This time frame is optimal because before 11 weeks, the fetus is still too small and its body structure not fully developed, making it difficult to distinguish pathological abnormalities.

However, after 14 weeks, the fluid accumulation under the fetal neck gradually disappears along with the nuchal translucency, making assessment impossible. A normal NT measurement is generally less than 3 mm. If the measurement exceeds 3 mm, it is considered increased NT thickness, indicating a higher risk of chromosomal abnormalities in the fetus, and amniocentesis should be promptly recommended.

In addition, although NT scans can be done anytime between 11 and 14 weeks, it is commonly recommended around week 12, when the fetus has reached an appropriate size and shape, allowing for more accurate measurement of the subcutaneous fluid thickness at the back of the neck.