Module 7: 2nd And 3rd Trimester Biometry Flashcards
For 2nd and 3rd trimester measurements what are the routine measurements? 7
- Biparietal diameter (BPD)
- Head circumference (HC)
- Abdominal circumference (AC)
- Femur length (FL)
- Humerus Length (HL)
- Occipital frontal diameter (OFD)
- Cephalic index (CI)
How accurate is Biparietal diameter compared to other 2nd trimester dating?
The most accurate measurement in the 2nd trimester for dating a pregnancy
Can we use Biparietal diameter measurements in the 1st trimester?
Yes
When can we use the Biparietal diameter (BPD)?
+/- 7 days on early 2nd trimester
What are some BPD landmarks? 3
- Falx cerebri
- Thalami in the middle of the brain
- Cavun septi pellucidi
The BPD land marks are what kind?
Intracranial landmarks
What is the Falx cerebri? 2
- Fibrous structure that separates the cerebral hemispheres
- Visualized as a horizontal line - anterior to posterior
What is the thalami in the middle of the brain?
Paired hypoechoic structures that appear heart- shaped and lie on either side of the 3rd ventricle
What kind of landmark is the Cavum septi pellucidi? What does it look like?
- Anterior landmark in the brain.
- Hypechoic box
What is the blue box?
Cavum Septi Pellucidi
What do we need to remember about getting the CSP?
Make sure that we don’t get the box not the equal signs.
What is the BPD measurement criteria? 4
- Perpendicular to Falx (as possible)
- Symmetric hemispheres
- Measure at the widest point across the skull
- Calibres placed from outer bone edge to inner bone edge on the opposite side
leading edge to leading edge
What does leading edge to leading edge look like?
Label the structures
- Anterior horn of lateral ventricle
- 3rd ventricle
- Anterior occipital horn with choroid plexus (closest to transducer)
- Posterior occipital horn with choroid plexus (farthest from
transducer)
What are some BPD pitfalls? 3
- Head not perpendicular
- Cerebellum
- CSP not well seen
Label the structures
- The Falx is the structure that separates the straight line
- The CSP is the box
- The Thalami is the farthest left box
Where is the Head circumference (HC) measurement taken? What kind of caliper is used? How should we use it?
- Measured at the same level as the BPD
- Measured with an ellipse calliper
- Eclipse should be placed tightly around the outer skull (do not include skin)
What kind of landmarks are we looking for in terms of the Head Circumference?
Same landmarks as BPD
Occipital frontal diameter (OFD) can be done in conjunction with what?
BPD
OFC is obtained at what level?
Same level as BPD
How should measure OFD?
Measure from outer margin of the frontal bone to the outer marginal of the occipital bone
BPD and OFD are used to calculate what?
Cephalic index (CI)
What is the formula for Cephalic index (CI)?
CI = (BPD/OFD)(100)
What is the normal range for Cephalic index?
75% to 85%
The BPD is considered accurate for determining fetal age if the Cephalic index falls within range
What is two types of Cephalic index?
- Dolichocephalic
- Brachiocephalic
What is a dolichocephalic head?
<75% (narrow head)
What is a brachycephalic head?
> 85% (wide head)
Label the two Cephalic index heads from left to right
Left: Brachycephalic
Right: Dolichocephalic
What are Brachycephalic do to the Gestational age?
Overestimates it
What does the Dolichocephalic do for the Gestational Age?
It underestimates it
What does the brachycephalic measure do in terms of BPD?
It renders the BPD less accurate
What does the dolichocephalic do in terms of the BPD?
It renders the BPD less accurate
How do we calculate head circumference?
BPD and OFD are used to calculate head circumference.
HC = (BPD + OFD) x 1.57
How do we calculate Head circumference on the machine?
The machine does it automatically, they will have a ellipse calliper to measure head circumference
How much more is HC dependant on shape compared to BPD?
Less dependent on shape than the BPD
What is the abdominal circumference level?
Transverse plan of the fetal abdomen at the level of the fetal
What are some landmarks we look for with AC? What kind of shape should the AC look like? What should the ribs look like? 5
- Stomach
- Umbilical vein draining into portal sinus
- Round shape
- Rib Symmetry
- Adrenal glands (not kidneys)
What is this?
The Abdominal circumference
What are the AC measurement Criteria? 3
- Calliper are placed around the outer surface of the skin
- Includes fat, skin, and muscle
- Machines use an ellipse for the AC measurement
How many measurements do we need for the AC measurement method
At least three measurements
What is the formula for AC?
AC = AP diameter + TRX diameter x 1.57
With AC, how would the Umbilical vein be visualized? Which level would we measure AC? How would the kidneys be visulized?
- Umbilical vein would be visualized anteriorly
- Is a good AC
- Kidneys would be visualized posterior on this image
Label the structures
What are fetal abdominal structures identified on US? 5
- Liver
- Kidney
- Gallbladder
- Vasculature
- Urinary bladder
What are some less frequently demonstrated abdominal structures? 2
- Spleen
- Pancreas
The head and abdomen grow at what rates?
Different rates
What is the size difference on the head and abdomen before 32 weeks?
Head > abdomen
What is the head to abdomen ratio between 32 - 36 weeks?
Head = Abdomen
What is the head to abdomen ratio after 36 weeks?
Head < abdomen
Femur length correlates well with what and when? Why?
Gestational age between 14 and 22 weeks (less accurate later in pregnancy)
In terms of accuracy how does Femur length compare to BPD and HC?
Not as accurate
Why is it important to get femur length? 3
To look for
1. Skeletal abnormalities
2. Chromosomal abnormalities
3. Short for gestational age is abnormal
What are Femur length measurement criteria? 3
- Femur must be perpendicular to sound beam
- Measure femur closest to the transducer
- Both ends of bone clearly visible
Where should we place the calliper for femur length?
Callipers should be placed at each end bisecting the shaft
When measuring the femur if it looks bent what should we do?
Use a different window
Where on the femur should we measure?
The diaphysis (shift) do not include epiphysis
When are 2nd trimester additional measurements done?
18-20
Routinely performed at the detailed anatomy scan
What are additional things that we measure during the 2nd trimester? 6 (Besides biometry)
- Cerebellum size
- Nuchal fold
- Cisterns magna size
- Ventricular size
- Binocular distance
- All long bones
Cerebellum size correlates with what?
Gestational age
What is the range of the diameter for cerebella’s measurement?
14 to 22 is the diameter in mm that corresponds to the gestational age
What plane is the cerebella taken? What is a landmark of the cerebella’s measurements?
- Trans- cerebellielar
- Cavum septi Pellucidi (CSP)
What are some criteria for Cerebeullar measurements? How should we measure it?
- Balanced, symmetrical cerebellum
- Measures from other echogenic edge to ethogenc edge
What is nuchal folds
Measurement of the space between the occipital bone and outer skin surface
When is the measurement of the Nuuchal fold thickness done?
Measured from 16 - 20 weeks
What plane is Nichal fold measured as?
Same plane as cerebellum
What is the range size of the nuchal fold thickness?
> 6mm is abnormal
What percentage of fetus with Down syndrome will have a thickened nuchal fold
45%
What is the cisterns magna?
Measurement of the hypoechoic space between the edge of the cerebellum to the inner occipital bone
What plane is Cisterna Maga taken at?
In some plane as cerebellum and nuchal fold
What is a landmarks of the transcerebellar plane?
Cavum septi Pellcida
What is the criteria for cisterna magna measurements?
Measure Measure from edge of cerebellum to inner occipital bone
inner to inner
What is the upper limit of normal for a cisterna magna measurement?
1cm or 10 mm
Does Cisterna Magna change with gestational age?
Nope!
Seeing the Cisterna magna rules out what?
90% of spinal defects
Where are ventricular measurements done?
They are measured at the atrium of the lateral ventricle (trigone)
What plane is ventricular measured at?
Slightly more Cephalic than BPD
Why do we take ventricular measurements?
Assess for hydrocephalus
What plane is ventricular measurements taken at? What is a landmarks for ventricular measurements? 2
- Trans-ventricular plane
- Cavum Septo Pellucidi
What are some criteria for the ventricular measurements? 2
- Measure ventricle farthest from the transducer
- Measure at the level of the parietal occipital fissure
What landmark is ventricular measurement done?
Parietal occipital fissure
What is the upper limit of normal of parietal occipital fissue?
1cm or 10 mm
Does ventricular measurement change with GA?
No
What is binocular distance (orbital)?
Transverse image through both orbits as well as the Brancusi
How should we measure for occipital?
Outer orbit to outer orbit
What does biocular distance detect?
Hypotelorism and hyperteloriusm
For binocular distance, the normal distance changes with what?
Gestational age
What do we do in cases where FL or HL are measuring small?
Other long bones can also be measured
Why do we measure long bones?
Assess for skeletal dysplasia or aneuoloidy
What are some other skeletal measures we take? 2
- Foot length
- Nasal length
Can foot length be used for gestational age?
Yes when long bones are shortened
For gestational size compared to dates, the biometry is plotted on a graph to do what?
Assess gestational size for gestational age
The biometry is plotted according to the estimated gestational age by what?
LMP or an early ultrasound
Measuring small or large for dates are reasons to have what?
an ultrasound assess the fetal size
The ultrasound machine can configure an estimated fetal weight if what4 things are done?
4 biometry measures are done
1. BPD
2. HC
3. AC
4. FL
What is considered large for gestational age?
Biometry > 90%
What would classified as large for gestational age?
Biometry > 90%
What is a common reason for a biometry to large?
Gestational diabetes
What is macrosomia?
Weight > 4000 grams
Macrosomias have increased morbidity and mortality because of what? 2
- They have difficulties in delivery
- The newborn can become hypoglycaemic post delivery if mom had uncontrolled GDM
What is considered small for gestational age?
Biometry < 10%
What is IUGR?
Intrauterine growth restriction
Symmetric IUGR vs Asymmetric IUGR: what is the basic difference ?
Asymmetric vs symmetric
Symmetric IUGR vs Asymmetric IUGR: body measurements?
Head normal size but abdomen small vs All measurements are small
Symmetric IUGR vs Asymmetric IUGR: abnormalities due to what?
Placenta insufficiency vs chromosomal abnormalities
Symmetric IUGR vs Asymmetric IUGR: frequency
75% vs 25%
Symmetric IUGR vs Asymmetric IUGR: trimester occurrence rate?
Occurs in 3rd trimester vs occurs in 2nd trimester
Symmetric IUGR vs Asymmetric IUGR: occurrence rate?
Occurs in 3rd trimester vs occurs in 2nd trimester
Symmetric IUGR vs Asymmetric IUGR: occurrence rate?
Occurs in 3rd trimester vs occurs in 2nd trimester
