First Trimester Examination of the Fetal Nasal Bone
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The findings of recent studies suggest that examination of the
fetal profile at 11-13+6 weeks could have major beneficial implications
in screening for Trisomy 21 by maternal age and fetal nuchal translucency
(NT). However it is imperative that, as for the NT scan, sonographers
undertaking risk assessment by examination of the fetal profile
must receive appropriate training and certification of their competence
in performing the nasal bone scan.
This document describes the Fetal Medicine Foundation:
- Protocol for the assessment of fetal nasal bone
- Clinical application of nasal bone findings
- Certification in the first trimester assessment of the fetal
nasal bone
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Protocol for the assessment of fetal nasal bone
1. The gestational period must be 11 to 13+6 weeks - the nasal
bones first appear at a crown-rump length of 42 mm and increase
linearly with gestation.
2. The magnification of the fetus must be such that only the head
and upper thorax are present on the screen
3. A mid sagittal section of the fetal profile must be obtained
with the ultrasound transducer held parallel to the direction of
the nose. The ultrasound transducer should be gently tilted from
side to side to ensure that the nasal bone is seen separate from
the nasal skin.
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4. The echogenicity of the nasal bone should be greater that
the skin overlying it. In this respect, the correct view of the
nasal bone should demonstrate three distinct lines:
• The first two lines, which are proximal to the forehead,
are horizontal and parallel to each other, resembling an “equal
sign”. The top line represents the skin and bottom one,
which is thicker and more echogenic than the overlying skin,
represents the nasal bone.
• A third line, almost in continuity with the skin, but
at a higher level, represents the tip of the nose.
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Absence of the bottom line of the equals sign represents the
absence of the fetal nasal bone
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When the nasal bone line appears as a thin line, less echogenic
than the overlying skin, it suggests that the nasal bone is not
yet ossified, and it is therefore classified as being absent
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Clinical application of nasal bone findings
The incidence of an absent nasal bone is related to NT, CRL and
ethnic origin as well as aneuploidy, being more common with increased
NT, smaller CRL measurements and in fetuses of Afro-Caribbean parents.
Therefore it is not possible to give simple numbers by which the
presence of the nasal bone will reduce the risk for Trisomy 21
and the absence will increase the risk. The Fetal Medicine Foundation
software includes the nasal bone in the calculation of risks but
access it available only to sonographers who have been appropriately
trained and accredited in the assessment of the fetal nasal bone.
The software firstly calculates a risk based on maternal age,
fetal nuchal translucency and maternal serum free ß-hCG
and PAPP-A. If the risk is more than 1 in 50 and the nasal
bone is present then the risk does not change. If the risk
is 1 in 50 to 1 in 1,000 and the nasal bone is present the
risk is usually reduced. If the nasal bone is absent the risk
is always increased. The difficulty is in a patient with a
low risk result from NT (with or without serum biochemistry)
when the nasal bone appears to be absent. In this cases our
advice is that the patients are rescanned in one week and action
is only taken at that point if there is persistence of the
absence of the nasal bone.
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Certification in the first trimester assessment of the fetal
nasal bone
The requirements for certification in first trimester assessment
of the fetal nasal bone are:
• FMF certification in the 11-13+6 weeks scan
• Participation in the FMF audit of the distribution of
NT measurements and images and demonstration of satisfactory
performance
• Attendance of a recent FMF 11-13+6 Weeks Scan course, including
a lecture on the assessment of the fetal nasal bone
• Submission of a logbook of 5 images demonstrating the
presence (or absence) of the fetal nasal bone
Please send your completed logbook images to:
Nasal bone certification
The Fetal Medicine Foundation
137 Harley Street
London
W1G 6BG
Tel +44 (0)20 7034 3070
Fax +44 (0)20 7034 3071
Email: certification@fetalmedicine.com
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