Second Trimester Absent or Hypoplastic Nasal Bone: Counseling and Management
The nasal bone is imaged on prenatal ultrasound in the second trimester of pregnancy, evaluated perpendicular to the longitudinal axis of the nose, in the mid sagittal plane of the fetal face. A hypoplastic nasal bone can be defined as a ratio against the biparietal diameter, by absolute length, by gestational age-based percentiles or by multiples of the median. If absent or hypoplastic nasal bone is suspected, next steps should include detailed anatomic evaluation and aneuploidy testing. SMFM considers an absent or hypoplastic nasal bone a “soft marker” which is a “minor ultrasound finding” associated with an increased risk of aneuploidy.
Isolated Ultrasound Finding
Offer aneuploidy testing
NIPS or quad screening if NIPS is unavailable or cost-prohibitive or
Diagnostic testing via amniocentesis
If negative quad serum screening
Offer counseling to estimate the residual risk of Trisomy 21 to inform discussion of following options
No further evaluation | cfDNA | Diagnostic testing
If negative NIS screening
No further evaluation is recommended
Absent or hypoplastic nasal bone occurs in 0.1 to 1.2% of euploid pregnancies. It is associated with trisomy 21, especially when occurring with other structural anomalies or soft markers. If screening is low risk for aneuploidy, this finding is of little clinical significance and most likely a normal variant.
Trisomy 21 risk is increased by a factor of 6.6 when isolated absent or hypoplastic nasal bone is seen
No further additional antenatal ultrasound or fetal surveillance is recommended with negative NIPS screening
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