Pregnancy topic

Nuchal Translucency (NT)

The 11–13+6 week chromosomal screen — timing, normal range, results

The nuchal translucency (NT) scan is an ultrasound performed at 11–13+6 weeks that measures fluid behind the fetal neck. Combined with maternal age and blood markers (β-hCG, PAPP-A), it estimates the risk of trisomy 21, 18, and 13 — a risk estimate, not a diagnosis.

Timing

Window: 11 weeks 0 days – 13 weeks 6 days.
Why: NT thickness is most informative here. After week 14 fluid drains.
Best CRL: 45–84 mm (typically weeks 12–13).

Normal NT thickness

NT grows with gestational age and CRL, so the cutoff is age-adjusted.

Week 11: ~1.6 mm (95th percentile 2.4 mm).
Week 12: ~1.8 mm (95th 2.7 mm).
Week 13: ~2.0 mm (95th 2.9 mm).
General cutoff: ≤ 3.0 mm is reassuring.
> 3.0 mm: further testing usually offered.
> 3.5 mm: clearly elevated chromosomal and cardiac risk.

Reading the risk number

Results are reported as "1/N" — the chance of an affected pregnancy. NT alone is rarely used; it is combined with hormones for the first-trimester combined screen.

Low risk: better than 1/1,000 — usually no further testing.
Borderline: 1/250 – 1/1,000 — NIPT may be offered.
High risk: 1/250 or worse — NIPT or diagnostic testing (CVS/amnio) offered.

After a high-risk result

NIPT (cell-free DNA): maternal blood test, 99%+ accuracy. No procedure risk.
CVS (weeks 10–13): diagnostic; ~1/500 miscarriage risk.
Amniocentesis (16–20 wk): most definitive; ~1/900 miscarriage risk.
Anatomy scan (18–22 wk): structural assessment.

Is the test required?

Screening is offered, not mandatory. NT is non-invasive and quick (about 30 minutes), so most clinicians recommend it, but family history, maternal age, and prior pregnancy history factor into the decision.