Fetal biometry

Corect measuring of fetal biometry

1. Fetal Fetal
Biometry Biometry
Fetal Fetal
Biometry Biometry
Dr Sam Ofori
KF Medical
04/2010

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

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

4. 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 ca lculations of the Fetal FetalBiometry Biometry
can be obtained by ellipse function (the machine ca lculations of the ellipse must be accurately set to ensure they compl y 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).

5. Standard Sections for FetalBiometry BMUS have recommended charts by Chitty et al, 1994 for all
measurements except for crown rump length (CRL).
BiparietalDiameter (BPD) & Head Circumference (HC):
Find the long axis of the fetus, then angle the pro be through 90º
to this axis. Angle so that the beam is along the t ransverse plane
through the fetal head. Check for symmetry of the he ad and
Fetal FetalBiometry Biometry
ensure the shape appears oval.
Using Altman & Chitty (1997) charts the landmarks s hould 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 cho roid plexus visible

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

7. Section of Campbell & Thoms (1977), used by Chitty et al (2006): Fetal FetalBiometry Biometry

8. HC Fetal FetalBiometry Biometry

9. Abdominal Circumference: F
Find the long axis of the fetus (L/S of the aorta)
F
Rotate the transducer through 90º
F
Move up/down to locate a T/S at level of the umbili cal vein
Landmarks required:
F
Circular section to include fetal spine and descendi ng aorta
F
Short section of the umbilical vein in anterior thi rd (along imaginary line between anterior abdominal wall &
fetal
spine)
Fetal FetalBiometry Biometry
line between anterior abdominal wall &
fetal
spine)
F
Stomach bubble

10. Section used to measure the abdominal circumference: Fetal FetalBiometry Biometry

11. Fetal FetalBiometry Biometry

12. Measurement Technique: π
Use the ellipse function
π
Measure 2 diameters anterior-posterior (d1) and tra nsverse
diameter (d2). π
Calculate using AC = π(d1 + d2)/2
Fetal FetalBiometry Biometry

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

14. Section for measuring the femur length (Chitty, et al 2006): Fetal FetalBiometry Biometry

15. The femur is located by scanning through the fetus i n 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 measurin g 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 pa rallel with the
transducer and posterior shadowing should be seen behind the
femur, to get a good section with the end points cl early defined. Other Fetal FetalBiometry Biometry
femur, to get a good section with the end points cl early defined. Other authors suggest a slight angulation of the femur, to see the end points
better. What is important is to ensure the femur i s close to parallel,
otherwise you will get foreshortening. Dudley & Smi th (2003) suggest
that as long as the angle is between 0º and 35º the re is a little
difference in the measurement, but recommend as near to parallel as
possible to ensure standardisation of the technique .

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

17. F
Ear length (not common – measured in some research)
F
Nasal bone
F
Thorax size
F
Renal size Thigh size
Fetal FetalBiometry Biometry
F
Thigh size
F
Placental thickness

18. Va/Hem & Vp/Hem Fetal FetalBiometry Biometry

19. TCD, CM, CSP, CPC Fetal FetalBiometry Biometry

20. Fetal FetalBiometry Biometry

21. TCD Fetal FetalBiometry Biometry

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

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

24. 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, sm all / large for
dates.GA is the known variable – so should be placed on the X -axis
of the graph.
Fetal FetalBiometry Biometry

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

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

27. Factors affecting measurement Accuracy:
1. Operator error (intra-observer & inter-observer error)
2. Calliper inaccuracy
3. Poor image resolution (difficulties defining ma rgins)
4. Variable landmarks for section
5. Technical limitations:
- maternal habitus
- fetal activity
Fetal FetalBiometry Biometry
- 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

28. References
Altman, D.G., & Chitty, L.S. (1994). Charts of feta l 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
Fetal FetalBiometry Biometry
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 Fet al Size: 3.
Abdominal Measurements. British Journal of Obstetrics &
Gynaecology, 101, 125 – 131.

29. Chitty, L.S., & Altman, D.G. (1994c). Charts of Fet al 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 FetalBiometry Biometry