Fetal Hypoplastic Nasal Bone Diagnosis and Management

Definition and Measurement

A hypoplastic nasal bone in the second trimester is defined by multiple measurement criteria, including a biparietal diameter-to-nasal bone ratio ≥10 or 11, absolute length <2.5 mm, gestational age-based percentiles <2.5th percentile, or multiples of the median <0.75 or 0.7 MoM, with the latter demonstrating superior test characteristics for trisomy 21 detection, as recommended by the American College of Obstetricians and Gynecologists 1, 2, 3
The nasal bone must be imaged perpendicular to the longitudinal axis of the nose in the midsagittal plane of the fetal face for accurate assessment, according to the American Institute of Ultrasound in Medicine 1

Absent or hypoplastic nasal bone occurs in 0.1% to 1.2% of euploid pregnancies, with significant ethnic variation, as noted by the American College of Obstetricians and Gynecologists 2, 4
Significant ethnic variation exists, with up to 9% prevalence in Afro-Caribbean populations, though no difference has been demonstrated between White and Asian populations, as reported by the National Institute of Child Health and Human Development 2, 5

The association between absent or hypoplastic nasal bone and trisomy 21 is well-established, with a positive likelihood ratio of 23 when combined with other markers, but only 6.6 when isolated, as stated by the American College of Medical Genetics and Genomics 2, 6
For patients with isolated absent or hypoplastic nasal bone and no prior screening, the American College of Obstetricians and Gynecologists recommends counseling regarding trisomy 21 probability and offering noninvasive aneuploidy screening through cell-free DNA or diagnostic testing via amniocentesis based on clinical circumstances and patient preference 2, 3, 4
For patients with negative serum screening and isolated absent or hypoplastic nasal bone, the American College of Obstetricians and Gynecologists suggests counseling regarding trisomy 21 probability and discussing options including no further evaluation, noninvasive screening via cell-free DNA, or diagnostic testing via amniocentesis 2, 7
For patients with negative cell-free DNA screening and isolated absent or hypoplastic nasal bone, the American College of Obstetricians and Gynecologists recommends no further aneuploidy evaluation, as this finding is most likely a normal variant in the presence of low aneuploidy risk 1, 2, 6

No additional antenatal ultrasound surveillance is recommended for isolated hypoplastic nasal bone alone, as stated by the American Institute of Ultrasound in Medicine 6
Careful evaluation of fetal anatomy is warranted, as absent nasal bone can be associated with other genetic syndromes beyond common aneuploidies, according to the National Institute of Child Health and Human Development 1, 6
The American College of Obstetricians and Gynecologists advises against recommending invasive testing solely for isolated hypoplastic nasal bone after negative cell-free DNA screening 8
The American College of Obstetricians and Gynecologists also advises against overlooking ethnic background when interpreting this finding, as normal variant rates differ substantially 2

The American College of Radiology recommends that the fetal nasal bone be routinely visualized during the second‑trimester anatomic survey, which is typically performed between 18 and 22 weeks gestation. 9
According to the American College of Radiology, a normally ossified nasal bone identified at 22 weeks should be documented as part of the routine anatomic survey without additional follow‑up. 9

The American College of Medical Genetics and Genomics (as reported in Genetics in Medicine) notes that nasal bone assessment is most commonly employed in the first trimester (11–14 weeks), where absence of the nasal bone serves as a strong marker for trisomy 21. 10
The American College of Radiology states that in the second trimester (19–22 weeks) nasal bone hypoplasia remains a valid soft marker for chromosomal abnormalities, but the presence of a normal nasal bone at this stage is the expected finding in the vast majority of fetuses. 9

The American College of Radiology guidelines emphasize that soft markers such as nasal bone assessment have reduced clinical relevance in the current era of first‑trimester screening and cell‑free DNA testing, and a normal nasal bone at 22 weeks simply confirms normal fetal development without necessitating special consideration. 9

American Journal of Obstetrics and Gynecology, 2021

Journal of the American College of Radiology, 2021

Genetics in Medicine, 2009