Fetal biometry

FetalFetal BiometryBiometryFetalFetal BiometryBiometry
Dr Sam Ofori
KF Medical
04/2010

Fetal Biometry - is the measurement of parameters to
assess gestational age and growth
Dating:
Why date Pregnancies?
Measurements:
Mean gestational sac diameter (prior to seeing embryo)
FetalFetal BiometryBiometry
Gestational sac volume (prior to seeing embryo)
CRL 11-13+6 weeks if NT(NSC recommends 8+ weeks)
Head circumference (HC) 13+ weeks
Femur length (FL) 13+ weeks
After 25 weeks gestational age calculations become less accurate,
so serial growth scans should be performed two weeks apart to
assess growth profile.

Other measurements can be taken, these include humerus (same as
femur – useful in some types of Dwarfism), transverse cerebellar
diameter, foot, orbits, mandible etc.
Growth:
Ultrasound is used to assess changes in fetal head and body size,
with increasing gestational age to allow detection of abnormal growth
patterns.
Why assess growth?
Which measurements?

Measurement technique:
Circumference measurements can be either plotted (traced), using
callipers to trace around the circumference or Derived. Derived can
be by using a point method (join the dots freehand), measuring 2
diameter and calculating the circumference or using the equipment’s
ellipse setting.
BMUS (2006) recommend the use of derived measurements & these
can be obtained by ellipse function (the machine calculations of the
can be obtained by ellipse function (the machine calculations of the
ellipse must be accurately set to ensure they comply with BMUS
recommendations) or by manual calculations from the BPD (outer to
outer edge of the parietal bone) and OFD measurements (outer edge
of the frontal bone to outer edge of the occipital bone).

Standard Sections for Fetal Biometry
BMUS have recommended charts by Chitty et al, 1994 for all
measurements except for crown rump length (CRL).
Biparietal Diameter (BPD) & Head Circumference (HC):
Find the long axis of the fetus, then angle the probe through 90º
to this axis. Angle so that the beam is along the transverse plane
through the fetal head. Check for symmetry of the head and
ensure the shape appears oval.
Using Altman & Chitty (1997) charts the landmarks should include:
1. Oval/Rugby ball shaped
2. Midline falx, broken by cavum septum pellucidum
3. Anterior horns of the lateral ventricles
4. Posterior horns of the lateral ventricles, with choroid plexus visible

Measurements should be:
OUTER-OUTER edge of the parietal bone to get the BPD
OUTER edge of the frontal bone to OUTER edge of the occipital
bone to get the OFD (Occipito-frontal diameter)
Calculations should be made using the formula: HC = π(BPD +
OFD)
Alternatively, the ellipse function can be used around the outer edge
of the fetal skull

Section of Campbell & Thoms (1977), used by Chitty et al (2006):

HC

Abdominal Circumference:
Find the long axis of the fetus (L/S of the aorta)
Rotate the transducer through 90º
Move up/down to locate a T/S at level of the umbilical vein
Landmarks required:
Circular section to include fetal spine and descending aorta
Short section of the umbilical vein in anterior third (along imaginary
line between anterior abdominal wall & fetal spine)
line between anterior abdominal wall & fetal spine)
Stomach bubble

Section used to measure the abdominal circumference:

Measurement Technique:
Use the ellipse function
Measure 2 diameters anterior-posterior (d1) and transverse
diameter (d2).
Calculate using AC = π(d1 + d2)/2

Femur Length:
Find the long axis of the fetus
Identify femur at caudal end
Rotate transducer until whole of femur is seen, as close to
horizontal as possible (reduce chance of foreshortening of femur)
Landmarks:
Full length of femur
Clear blunt ends
Clear blunt ends
Soft tissue visible beyond both ends

Section for measuring the femur length (Chitty, et al 2006):

The femur is located by scanning through the fetus in T/S in a caudal
direction to the iliac crest to locate a transverse view of the femur.
slowly rotate the probe through 90º and ensure the maximum length
is seen (This will ensure that you are not measuring the humerus,
which is a common mistake when you start scanning). Freeze image
on screen and measure.
Some literature suggest that the femur should be parallel with the
transducer and posterior shadowing should be seen behind the
femur, to get a good section with the end points clearly defined. Other
femur, to get a good section with the end points clearly defined. Other
authors suggest a slight angulation of the femur, to see the end points
better. What is important is to ensure the femur is close to parallel,
otherwise you will get foreshortening. Dudley & Smith (2003) suggest
that as long as the angle is between 0º and 35º there is a little
difference in the measurement, but recommend as near to parallel as
possible to ensure standardisation of the technique.

Measurement:
Linear measurement from one end of the femoral diaphysis to the
other
NOTE: the distal epiphysis ossifies in the last month of pregnancy,
take care not to include this in measurements at later gestations
Other measurements:
Cerebellar diameter (mm) = gestational age in weeks
Ventricles: posterior horn of lateral ventricle, measuring the
Ventricles: posterior horn of lateral ventricle, measuring the
ventricular atrium
Cystena magnum
Nuchal fat pad/ nuchal fold
Orbits
Long bones i.e. Humerus, radius, ulna etc
Feet

Ear length (not common – measured in some research)
Nasal bone
Thorax size
Renal size
Thigh size
Thigh size
Placental thickness

Va/Hem & Vp/Hem

TCD, CM, CSP, CPC

TCD

What to look for when choosing charts:
Date of study
Equipment used
Frequency of transducer
Criteria for an acceptable image (what landmarks for correct section
& calliper placement
Patient selection criteria
Number of patients
Number of patients
Number of scans per patient (longitudinal or cross-sectional study)
Range of gestational ages used and number of scans at each
gestational age
Whether numerical tables of data and the equation were published
Statistical method used
Is it a dating chart or a growth chart / graph?

CHARTS:
Dating graphs – should use cross-sectional data – 1 measurement
From each fetus (allows generalisation to be made about population
as a whole). A large sample is needed (100 – 500) and similar number
of measurements at each gestational age (GA).
GA is the unknown variable & should be placed on the Y-axis of the
graph.

Growth graphs – longitudinal study – serial measurements of the
same fetus. Excluding abnormal pregnancies (e.g. maternal diabetes,
Hypertension), but NOT the ones that are normal, small / large for
dates. GA is the known variable – so should be placed on the X-axis
of the graph.

Whose Charts?
BMUS recommends charts in their booklet “Clinical Applications of
Ultrasonic Fetal Measurements” but have since endorsed charts by
Chitty et al (1994 a,b,c), because they are statistically superior.
However, regardless of whose charts are used:
Very important to know the origin of the charts used (especially if
programmed into the machine)
- always use the correct technique that the author used to ensure
accuracy
- Ensure the correct section is used
- Use the correct charts for dating or growth
Make sure the image settings are optimal for each section
Regular QA checks to ensure accurate measurements
Plot the measurements yourself if manual plotting is the norm in
your dept., don’t leave them for someone in another dept. to plot

Criteria for measurements to assess fetal growth &
maturity:
Quick & easy to perform
Easily reproducible by any user
Well defined landmarks
Rapid increases in size over a wide gestational age
Small range of values for normal (standard deviation)
Applicable over a long period of the pregnancy
Charts available to cover the local population
Charts available to cover the local population
Correct charts for the calibration of the equipment used
Measurement error should not be significant
Preferably not affected by fetal abnormality
If used for dating – not affected by growth problems
Allows specific and early indications of IUGR or macrosomia if used
for growth

Factors affecting measurement Accuracy:
1. Operator error (intra-observer & inter-observer error)
2. Calliper inaccuracy
3. Poor image resolution (difficulties defining margins)
4. Variable landmarks for section
5. Technical limitations:
- maternal habitus
- fetal activity
- fetal lie
- abdominal compression of the fetus
- Fetal breathing movements
- Head moulding
- Multiple pregnancy
- Oligohydramnios – difficulties measuring AC, FL etc
- Large fetus – often unable to fit margins on screen

References
Altman, D.G., & Chitty, L.S. (1994). Charts of fetal size: Methodology.
British Journal of Obstetrics and Gynaecology, 101, 29-34.
Altman, D.G., & Chitty, L.S. (1997). New charts for ultrasound dating
of pregnancy. Ultrasound Obstet Gynecol, 10, 174-191.
Chitty, L.S. & Altman, D.G. (1994a). Charts of Fetal Size: 2. Head
Chitty, L.S. & Altman, D.G. (1994a). Charts of Fetal Size: 2. Head
Measurements. British Journal of Obstetrics and Gynaecology, 101,
35-43
Chitty, L.S., & Altman, D.G. (1994b). Charts of Fetal Size: 3.
Abdominal Measurements. British Journal of Obstetrics &
Gynaecology, 101, 125 – 131.

Chitty, L.S., & Altman, D.G. (1994c). Charts of Fetal Size: 4. Femur
Length. British Journal of Obstetrics and Gynaecology, 101, 132-135.
Dudley, N., & Brown, C. (2003). Fetal Femoral Angle: How important?
BMUS Conference Poster Presentation, December, 2003.
National Screening Committee 2004 Antenatal Screening, Working
Standards http://www.screening.nhs.uk/downs/workingstandards.pdf

Fetal biometry

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