Nuchal Fold Measurement Timing
The nuchal fold should be measured between 15 and 20 weeks of gestation, with a threshold of ≥6 mm defining an abnormally thickened nuchal fold during this gestational age window 1.
Key Technical Details
Measurement Window
- Primary measurement window: 15-20 weeks gestation 1
- The measurement can be extended through 24 weeks, though the standardized threshold of 6 mm was specifically validated for the 15-20 week period 1
Measurement Technique
The nuchal fold is measured in the transverse plane of the fetal head, angled caudally to capture:
- The cerebellum
- The occipital bone
- Calipers placed between the outer edge of the skin and outer edge of the occipital bone 1
Diagnostic Threshold
- ≥6 mm is considered thickened between 15-20 weeks 1
- This represents one of the most specific ultrasound markers for trisomy 21, with a positive likelihood ratio of 11-23 when associated with other findings, or 3.8 when isolated 1
Important Distinction: Nuchal Fold vs. Nuchal Translucency
Do not confuse nuchal fold with nuchal translucency:
- Nuchal translucency: First trimester measurement (11-14 weeks) 2
- Nuchal fold: Second trimester measurement (15-20 weeks) 1
These are distinct measurements performed at different gestational ages with different clinical implications.
Gestational Age Considerations
Research evidence suggests nuchal fold thickness increases with advancing gestational age 3, 4, 5, 6, 7, 8:
- At 16 weeks: mean approximately 3.1-3.7 mm 4, 5
- At 20 weeks: mean approximately 4.5-5.0 mm 4, 5
- At 24 weeks: mean approximately 5.1 mm, with 95th percentile remaining <6 mm 5
However, the SMFM guideline maintains the single threshold of ≥6 mm for the entire 15-20 week window 1, which simplifies clinical application and has been validated for trisomy 21 detection.
Clinical Pitfall
Some older research 6 suggested using differential thresholds (5 mm for 14-18 weeks, 6 mm for 19-24 weeks), but current SMFM guidelines standardize the threshold at 6 mm for 15-20 weeks 1. This unified approach reduces confusion and maintains high specificity for aneuploidy detection.