Nuchal translucency scan: timing and the cutoff week

What does the evidence say about nuchal translucency scan timing and cutoff during pregnancy?

1. International guidance described in a feasibility study places standard nuchal translucency measurement at a crown-rump length of 45 to 84 mm, corresponding to 11 weeks through 13 weeks and 6 days. 1
2. A prospective screening study performed nuchal translucency scans between 11 weeks and 13 weeks and 6 days using established first-trimester screening guidance. 2
3. A timing study found that scheduling nuchal translucency measurement around 12 to 13 weeks reduced both too-early appointments and missed measurements caused by a crown-rump length above the measurement limit. 3
4. Nuchal translucency plus maternal age is a screening approach for chromosomal differences, not a diagnostic result by itself. 4
5. After a negative cell-free DNA result, SMFM does not recommend an 11-to-14-week ultrasound solely to measure nuchal translucency. Ultrasound may still be needed for other pregnancy questions, so the reason for the examination matters. 5

Is nuchal translucency scan timing and cutoff safe in each trimester?

• First trimester. Nuchal translucency measurement is designed for the late first trimester. The appointment may need rescheduling if dating or crown-rump length falls outside the measurement window.
• Second trimester. After the nuchal translucency window closes, a clinician can review second-trimester anatomy ultrasound and other screening or diagnostic options based on timing and preferences.
• Third trimester. A nuchal translucency scan is not a third-trimester test. Later ultrasound questions should be discussed in the context of the pregnancy’s current clinical needs.