What does a nuchal translucency measurement of 1.2 mm with a visible nasal bone indicate regarding the risk of chromosomal aneuploidies?

A nuchal translucency of 1.2 mm with a present nasal bone indicates low risk for chromosomal abnormalities, particularly Down syndrome (trisomy 21).

Risk Assessment

Your findings are reassuring. A nuchal translucency measurement of 1.2 mm falls well below the threshold of concern (< 3.0 mm), which is defined as increased NT 1. The presence of the nasal bone further reduces the risk of aneuploidy, as nasal bone absence is a powerful marker for chromosomal abnormalities 1.

Key Context from Guidelines:

• Increased NT is defined as ≥ 3.0 mm or above the 99th percentile for crown-rump length 1
• At NT ≥ 3.0 mm, approximately one-third of fetuses will have chromosomal abnormalities, with half being trisomy 21 1
• NT alone detects only 70% of trisomy 21 cases, which is why it should be combined with maternal serum markers (PAPP-A and free β-hCG) for first-trimester combined screening 1

Nasal Bone Significance:

The presence of the nasal bone is highly reassuring. Research demonstrates that:

• Nasal bone absence occurs in approximately 41.9% of trisomy 21 fetuses 2
• When the nasal bone is absent, it serves as a highly predictive marker for trisomy 21 2
• In one study, nasal bone was absent in all three cases of trisomy 21, but present in 99.7% of normal fetuses 3
• Assessing nasal bone presence improves first-trimester screening algorithms by adjusting aneuploidy risk calculations 1, 4

Clinical Implications

With NT of 1.2 mm and present nasal bone:

1. No immediate concern for aneuploidy - both markers are favorable
2. Complete first-trimester combined screening by incorporating maternal serum markers (PAPP-A, free β-hCG) along with maternal age to calculate final risk 1
3. Consider cell-free DNA (cfDNA) screening - Current ACMG guidelines (2025) strongly recommend prenatal cfDNA screening over traditional methods for the general population for trisomies 21,18, and 13 5
4. Routine second-trimester anatomic survey remains indicated regardless of first-trimester results 1

Important Caveats

• NT measurement should never be used in isolation to determine risk due to inferior performance compared to combined screening 1
• First-trimester combined screening achieves approximately 90% detection rate for aneuploidy at a 5% false-positive rate 1
• NT is not a good screening test for structural anomalies or congenital heart disease when normal - only 50% of major anomalies can be detected before 14 weeks 1
• Even with normal NT, standard second-trimester anatomic survey at 18-22 weeks is essential, as sensitivity for anomaly detection is 75% at that gestational age versus only 50% in the first trimester 1

Bottom line: Your measurements suggest low risk, but complete the screening process with maternal serum markers or proceed directly to cfDNA testing per current guidelines, followed by routine second-trimester anatomic evaluation.