Management of Absent Nasal Bone at 20 Weeks Gestation
Direct Recommendation
For a fetus with isolated absent nasal bone at 20 weeks, your next step depends entirely on prior aneuploidy screening status: if no prior screening was performed, offer cell-free DNA testing or amniocentesis; if serum screening was negative, discuss options including cell-free DNA or amniocentesis based on residual risk calculation; if cell-free DNA screening was negative, provide reassurance and perform no further aneuploidy evaluation. 1, 2
Clinical Significance and Background
What This Finding Means
- Absent or hypoplastic nasal bone occurs in only 0.1% to 1.2% of chromosomally normal pregnancies, making it a relatively uncommon finding 2
- The association with trisomy 21 is well-established but varies dramatically based on whether the finding is isolated: the positive likelihood ratio is 23 when combined with other markers but drops to only 6.6 when truly isolated 2
- In a Chinese cohort study, isolated absent nasal bone carried only a 7.14% risk of abnormal karyotype, compared to 83.33% when associated with other structural abnormalities 3
Critical Ethnic Considerations
- Ethnic background profoundly affects interpretation: up to 9% of Afro-Caribbean fetuses have absent nasal bone as a normal variant, compared to 2.8% in Caucasians and 6.8% in Asians 4
- You must factor ethnicity into your counseling, as the baseline false-positive rate differs substantially across populations 2, 4
Management Algorithm Based on Prior Screening Status
Scenario 1: No Prior Aneuploidy Screening
Recommended approach:
- Perform a detailed anatomic ultrasound to confirm the absent nasal bone is truly isolated (no other soft markers or structural abnormalities) 1
- Counsel the patient on the probability of trisomy 21 using the likelihood ratio of 6.6 for isolated findings 2
- Offer noninvasive aneuploidy screening through cell-free DNA testing as the preferred option 1, 2
- If cell-free DNA is unavailable or cost-prohibitive, offer quad screen 1
- Alternatively, offer diagnostic testing via amniocentesis based on patient preference and clinical circumstances 1, 2
Scenario 2: Negative Serum Screening Already Performed
Recommended approach:
- Calculate the residual risk of trisomy 21 by applying the likelihood ratio of 6.6 to the post-screening risk 1, 2
- Given the variation in detection rates across serum screening methods, the residual risk may increase above thresholds where further evaluation is warranted 1
- Offer three options based on the calculated residual risk and patient preference: no further evaluation, cell-free DNA screening, or diagnostic amniocentesis 1, 2
- The decision hinges on whether the residual risk crosses the patient's personal threshold for pursuing additional testing 1
Scenario 3: Negative Cell-Free DNA Screening Already Performed
Recommended approach:
- Provide reassurance and perform no further aneuploidy evaluation 1, 2
- In the presence of negative cell-free DNA screening, this finding is most likely a normal variant of no clinical importance 1, 2
- No additional antenatal ultrasound surveillance is needed for this finding alone 1, 2
Additional Evaluation Requirements
Comprehensive Anatomic Assessment
- Perform careful evaluation of all fetal anatomy, as absent nasal bone can be associated with other genetic syndromes beyond common aneuploidies 1, 2
- Specifically look for structural heart defects, limb abnormalities, growth restriction, and other soft markers that would change the risk calculation 3
- The presence of additional abnormalities dramatically increases the risk of chromosomal abnormalities from 7% to over 80% 3
No Routine Follow-Up Imaging
- Do not recommend additional ultrasound surveillance for isolated absent nasal bone alone 1, 2
- Follow-up imaging is only warranted if other abnormalities are identified that require monitoring 1
Common Pitfalls and How to Avoid Them
Pitfall 1: Recommending Invasive Testing After Negative Cell-Free DNA
- Never recommend amniocentesis solely for isolated absent nasal bone when cell-free DNA screening is negative 2
- The negative predictive value of cell-free DNA is sufficiently high that this finding becomes clinically insignificant 1, 2
Pitfall 2: Ignoring Ethnic Background
- Failing to account for ethnicity leads to overestimation of risk in Afro-Caribbean populations where 9% prevalence is normal 2, 4
- Always document maternal ethnicity and incorporate this into your risk counseling 4
Pitfall 3: Failing to Distinguish Isolated vs. Non-Isolated Findings
- The risk stratification is completely different: 7% vs. 83% abnormal karyotype risk 3
- Perform a meticulous anatomic survey to ensure no other abnormalities are present before labeling the finding as "isolated" 1, 3
Pitfall 4: Inadequate Image Acquisition
- The nasal bone must be imaged perpendicular to the longitudinal axis of the nose in the midsagittal plane for accurate assessment 2
- Poor technique leads to false-positive findings and unnecessary patient anxiety 2
Strength of Evidence Discussion
The recommendations are based on high-quality 2021 SMFM guidelines published in the American Journal of Obstetrics and Gynecology 1, which provide GRADE 1B recommendations for management based on prior screening status. These guidelines supersede older protocols and reflect the current era of cell-free DNA screening dominance. The supporting research demonstrates consistent findings across multiple populations, though with important ethnic variations that must be considered 3, 5, 4.