Precautions Before NT Scan
Ensure the scan is scheduled between 11-14 weeks gestation (crown-rump length 45-84 mm) with accurate dating, preferably by ultrasound before the NT measurement, and verify the sonographer is certified through established quality assurance programs. 1, 2
Critical Timing Considerations
The optimal window for NT measurement is 12-13 weeks gestation when scheduling is based on last menstrual period (LMP) 3. This timing minimizes three key problems:
- Too early: Requires rescheduling if CRL <45 mm
- Too late: Measurement impossible if CRL >84 mm
- Missed miscarriage: Decreases significantly with advancing gestation
If prior ultrasound dating exists, schedule at the confirmed gestational age to avoid the 1.6% risk of missed miscarriage and 0.3% risk of being over-gestation at scan time 3.
Sonographer Certification Requirements
The sonographer must be certified through either:
Before clinical interpretation, sonographers should provide the laboratory with paired NT/CRL measurements from at least 30 pregnancies to validate appropriate reference data, ensuring the 95% confidence interval of median NT multiples of the median (MoM) falls between 0.9-1.1 1.
Quality Assurance Precautions
Ongoing monitoring is essential because considerable inter- and intra-observer variability exists in NT measurements 2. The laboratory must:
- Monitor NT data from each sonographer continuously
- Recalculate medians when epidemiological monitoring shows median NT MoM consistently outside 0.9-1.1 range
- Update center- or sonographer-specific medians annually 1
Patient-Specific Precautions
Confirm accurate gestational age before the scan - preferably by ultrasound rather than LMP alone. An ultrasound-confirmed gestational age before screening is optimal because CRL measurements provided during first-trimester screening reduce the need for result reclassification 1. This is critical because reclassifying screen-positive pregnancies as screen-negative (potentially converting true positives to false negatives) poses significant risk.
For patients with high body mass index: Be prepared for potential need for transvaginal approach, as transabdominal imaging may be suboptimal in approximately 5% of patients 2.
Technical Equipment Precautions
Avoid Doppler ultrasound during routine NT measurement when NT is normal or <3 mm, given the theoretical risk of thermal damage to the developing fetus from color and pulsed Doppler 2. Doppler should be reserved only for fetuses with NT ≥3 mm who require additional risk assessment 2.
Important Clinical Context
Do not perform NT scan solely for screening if cell-free fetal DNA testing is already negative 2. However, understand that NT measurement remains valuable in twin pregnancies where maternal serum markers cannot identify which fetus is at risk 2.
Common pitfall: A normal NT does not eliminate the need for invasive testing in high-risk women (age >35 or significant family history), as NT alone detects only 70% of trisomy 21 cases 2. NT should not be used in isolation for risk determination 2.