Nuchal Translucency at 21 Weeks: Not the Appropriate Measurement
Nuchal translucency (NT) is not measured at 21 weeks of gestation—this measurement is only valid between 11+0 and 14+0 weeks when the crown-rump length is 45-84 mm. 1, 2
Why NT Cannot Be Measured at 21 Weeks
Timing Window for NT Measurement
- NT measurement is crown-rump length dependent and loses all predictive value outside the 11-14 week window. 2
- The hypoechoic space between the overlying skin and underlying soft tissues of the posterior cervical spine that defines NT is only measurable and clinically meaningful during the first trimester. 2
- By 21 weeks, this anatomical finding has typically resolved or transformed into other structures, making measurement neither technically feasible nor clinically interpretable. 1
What Should Be Done at 21 Weeks Instead
At 21 weeks of gestation, the appropriate screening is the standard second trimester anatomic fetal survey, not NT measurement. 1
- The optimal time to assess the fetal heart is 18-22 weeks of gestation, which is when detailed anatomic evaluation should occur. 1
- Second trimester ultrasound at 18-22 weeks has approximately 75% sensitivity for detecting congenital anomalies, compared to only 50% at 11-14 weeks. 2
Normal NT Values (When Measured Correctly at 11-14 Weeks)
For reference, when NT is measured at the correct gestational age:
Normal Range
- Normal NT is defined as less than 3 mm or below the 99th percentile for crown-rump length. 1, 2
- NT increases physiologically with crown-rump length, so gestational age must be considered when determining if a measurement is abnormal. 2
Abnormal Threshold
- NT ≥3 mm or above the 99th percentile is considered increased and warrants immediate genetic counseling and invasive diagnostic testing. 1, 2, 3
- Approximately one-third of fetuses with NT ≥3 mm will have chromosomal abnormalities, with half being trisomy 21. 2, 3
Clinical Pitfall to Avoid
Do not attempt to measure or interpret NT measurements outside the 11-14 week window, as this provides no clinically useful information and may lead to inappropriate management decisions. 1, 2 If there were concerns about increased NT earlier in pregnancy that were not addressed, the patient should now undergo comprehensive second trimester anatomic survey and potentially fetal echocardiography if the earlier NT was ≥3 mm. 1