My name is Carol Bagnall and I am a sonographer working in women's health.
This presentation gives guidance on how to optimise the nuchal translucency examination.
Quality improvements to antenatal screening were introduced in NZ in 2010 when maternal
serum was combined with the nuchal translucency (NT) and crown rump length (CRL) measurements.
Previously maternal age and then NT were used to screen for Down syndrome and other conditions.
A high quality 11–13 week scan and precise NT and CRL measurements are essential in providing
women with an optimum risk result for antenatal screening.
There is a tendency to under measure the nuchal translucency.
It important the NT and CRL are measured correctly, due to the amplification of errors that can
occur when they are not measured precisely.
This presentation provides guidance and a refresher on best practice and includes tips
for optimising both the image and the measurements.
Note that different equipment will require further tailoring of the examination.
As part of 12 week scan we should document the early fetal anatomy and placental location.
This presentation is only about optimising the NT and CRL measurements.
The results of the maternal serum test (known as the MSS1) rely on an accurate CRL measurement.
The CRL is used to standardise the biochemistry.
If the CRL is under measured it may increase the chance of a false negative screen rate
and if it is over measured it is more likely to increase the false positive screen rate.
To obtain a precise CRL you need to take a sagittal view of the fetus in a neutral position
horizontal on the screen.
Callipers are to be placed on the crown which is the skin above the parietal bone and the
rump, which is inferior to the tip of the sacrum.
For antenatal first trimester screening, or MSS1, the CRL should be between 45 and 84
You need to use appropriate depth and magnification.
Ideally the scan is done around 12–13 weeks when the CRL is 56 mm or greater.
The measured CRL axis should be at 90 degrees to the ultrasound beam, measuring at angles
greater than 30 degrees will likely lead to measurement error.
This slide demonstrates the fetus lying in a horizontal position in a sagittal view with
the callipers placed correctly at the crown and the rump of the fetus.
This slide show a fetus lying in the correct position with the callipers placed at 30 degrees,
However, it would be more ideal to have the callipers placed on the horizontal with respect
This slide shows the fetus in correct sagittal position but where magnification has not been
used making it harder to correctly place the calipers.
The following three slides demonstrate how much variation in measurment can occur depending
on fetal position, thus stressing the importance of correct fetal position when measuring the
This slide shows the fetus in the correct position where appropriate magnification has
been used so a reliable CRL can be obtained.
This is the same fetus but with a poorly obtained CRL demonstrating under measurement of the
CRL at 49.8 mm, which is a difference of 7.7 mm from the previous slide.
This slide shows the same fetus but where the angle of the fetus with respect to the
ultrasound beam is greater than 30 degrees.
The fetus is also not in a sagittal position.
This demonstrates an under measurement of the CRL at 53.2 mm which is a difference of
4.3 mm from the first slide.
To obtain a good quality NT measurement, the fetus needs to be in a midline sagittal position
using the intra cranial landmarks of the diencephalon, mid brain and 4th ventricle.
The maxilla should not be visible.
You should use the highest frequency transducer available; this may be a linear probe a curve
linear or a trans-vaginal probe.
You need adjust the depth and magnification so that you demonstrate only the head and
upper thorax in the field of view.
Adjust the gain settings.
Adjust the dynamic range and place the focus on the correct location.
You need to measure on the line to on the line that define the NT and measure the widest
This slide shows NT measurement where appropriate magnification has been used.
The fetus lies in a midline sagittal position with the appropriate intra cranial landmarks
seen. You can see there is no maxilla present, the intra cranial translucency at the back
of the fourth ventricle is present and the amnion is visible at the back of the fetus.
To help guide you that the fetus is in the correct position, you should note that the
fetal palate should be between 30 and 60 degrees relative to the horizontal as shown in this
The fetal head should not be too flexed onto the chin, the nasal tip should be level or
above the anterior abdominal wall relative to the horizontal and there should be a pocket
of fluid at least equivalent to the size of the palate visible between the fetal chin
Shown here is how the same fetus can be imaged and differing NT measurements can be obtained.
The same frequency probe was used, but the fetus is in a slightly different position,
and the only other adjustment was the gain.
The high quality image is on the left, demonstrating an NT of 1.8 mm by adjusting the gain and
position you can see that the NT is 1.6 mm which has resulted in an under measurement
Here we show the difference in NT obtained using different magnification settings.
This is the same image taken where in the left hand side of the screen the appropriate
magnification is used and the NT measured 1.8 mm.
In the right hand image magnification was not employed and the NT was 1.5 mm resulting
in a under measurement of 0.3 mm.
Using different frequency settings can change the NT measurement.
On the left hand side a curve linear high frequency probe was used and the NT was measured
In the right hand image a 1.5 MHz curved array linear probe was used and the NT was measured
at 1.4 mm an under measurement of 0.4 mm.
In this slide the top left image was taken using a high frequency 4–8 MHz curved array
probe and the NT is measured at 1.8 mm. In the right sided image using the C1–5 MHZ
lower frequency probe with poor magnification the NT is measured at 1.2 mm.
An under measurement of 0.6mm.
Note how the intracranial landmarks are not well defined and caliper placement is poor.
This next slide demonstrates how a Transvaginal Vaginal scan can help when the fetus is in
The slide on the right shows that the fetus was in a difficult position and the NT was
unable to be obtained, the slide on the left is the same fetus using a TV scan where the
NT was obtained at 1.4 mm.
Operator and skill can affect the NT measurement obtained.
Junior staff, practitioners who perform NT less frequently and those needing to improve
the quality of their images should be provided with support and mentoring by practitioners
with the skill to obtain high quality images.
The image on the left demonstrates the NT obtained by someone who was less experienced
or requiring support in NT scanning.
The measurement taken here is 1 mm.
The image on the right is a high quality image of the same fetus obtained by a more skilled
practitioner, the measurement obtained is 1.8 mm, a 0.8 mm difference.
This is the same fetus, demonstrating the image on the left where the operator had not
taken a satisfactory CRL measurement and the CRL is over measured.
The image on the right the appropriate CRL is obtained and is measured at 62 mm.
There can be a degree of luck involved in trying to manoeuvre a fetus into the optimal
position for obtaining an accurate NT.
In the top left image a fetus is shown in a difficult position, the same fetus is shown
below this image and to the right where techniques have been used to manoeuvre the fetus.
Techniques employed are at the sonographer's discretion and depend on individual circumstances
on the day of examination and can include getting the woman to fill or empty her bladder,
rolling the woman from side to side, or delaying scanning by 30 minutes – 1 hour.
So, in conclusion, it is important to remember it is the little things that count in obtaining
a high quality nuchal translucency and crown rump length measurement.
To get the most reliable risk result for women we need to optimise the ultrasound image every
time; use high frequency transducers when possible; adjust settings: 1: gain, 2: magnification,
3: frequency, and 4: opt for TV scan when appropriate. We may need to delay scanning. Remember, if
you cannot get a good NT image then second trimester screening is available.