2nd & 3rd Trimester Biometry Flashcards
What are the 3 measurements that are taken in the first trimester?
- Mean sac diameter (MSD)
- Crown rump length (CRL)
- Nucheal lucency
When is the MSD used?
When the embryo is not identifiable
To compare embryo size to sac size
What do you have to ensure you see when measuring the MSD?
Double decidual sign
What weeks is this measurement used?
4-7 to 8 weeks
What is the formula for MSD?
Length + Width + Height / 3
What is the growth of the gestational sac daily?
1.1mm/day
What are the measurements for the YS and embryo on a EV scan?
YS- 8mm
Embryo- 16mm
What are the measurements for the YS and embryo on a TAS scan?
YS- 20mm
Embryo- 25mm
When is the CRL measurement used?
6-13 weeks
What is the most accurate measurement to predict gestational age?
CRL
How accurate is the CRL?
+/- 3 days
How do you measure the CRL?
Measure from tip of head to end of rump
Do you include the yolk sac in the CRL?
No
What position should the baby be lying in when obtaining the CRL?
Neutral position
When is early oligohydramnios measures?
5 1/2 - 9 weeks
What is the formula to calculate oligohydramnios?
MSD (mm) - CRL (mm)
When is the nuchal lucency test preformed?
Done b/w 11 and 14 weeks
When imaging for a nuchal test, do you take a longitudinal or short axis picture of the fetus?
Long axis
Do you magnify to see nuchal lucency?
Yes ++
What can you commonly mistake the nuchal lucency for?
Amnionic membrane
How do you measure the nuchal lucency?
On to on
Echogenic area thru hypo echoic fluid to echogenic skin
What are some indications of increased nuchal fluid? (5)
- Increased nuchal
- Chromosomal abnormalities
- Cardiac abnormalities
- Blood disorders
- Skeletal dysplasias
What are the routine measurements for the 2nd & 3rd semester? (7)
- BPD
- Occipital frontal diameter (OFD)
- Cephalic index (CI)
- Head circumference (HC)
- Abdominal circumference (AC)
- Femur length
- Femur weight
When trimester is the BPD the most accurate?
2nd
How accurate is the BPD within the early 2nd semester?
+/- 7 days
What are the intracranial landmarks for the BPD? (3)
- Falx cerebri (anterior and posterior)
- Thalami in the middle
- Cavum septi pellucidi anterior;y
How does the falx appear in the BPD measurement?
Should create a flat line across the screen
How should your markers be positioned for the BPD measurement?
Outer to outer
Label the image on pg 18..
See pg 18
What is the name of the additional ventricle positioned posterior to the cavum septi pellucidi?
Cavum Verge
What does the Cavum verge fill with?
Nothing- eventually closes
What part of the head do you measure for the BPD?
Widest section of head
What other measurement is taken at the same level/conjunction as the BPD?
Occipital frontal diameter (OFD)
What are the BPD and OFD used to calculate?
Cephalic Index
How should your markers be positioned for the OFD?
Outer margin of the frontal bone to the outer margin of the occipital bone
What influences the BPD estimation of fetal age?
Head shape
What is the formula used to calculate cephalic index?
CI = BPD/OFD X 100
What is the normal range for the cephalic index?
75-85%
What is the shape of a dolichocephalic head?
Narrowed head
What % of fetuses have a dolichocephalic head?
< 75%
What is the shape of a brachycephalic head?
Wide head
What % of fetuses have a brachycephalic head?
> 85%
What 2 calculations can be used to calculate the HC?
BPD and OFD
What is the formula for HC?
HC= (BPD+OFD) X 1.57
How is the BPD measured in reference to calculating the HC?
Outer to outer
Is the HC less or more dependent of the shape of the head then the BPD?
Less dependent
What is the formula for the AC?
AC = AP diam + TRV diam X 1.57
Where are the calipers placed when measuring the AC?
Around outer skin
What are the AC landmarks? (6)
- Umbilical vein driving into portal sinus
- Stomach
- Round in shape
- Adrenal glands
- Ribs
- 3 ossification centres of the spine
What is the “hockey stick sign”
Umbilical vein draining into the portal sinus
Look at the planes through the abdomen on pg 33…
Refer to pg 33…
What are the fetal abdominal structures easily identified?
Liver, kidneys, GB, vasculature and UB
What abdominal structures are less frequently identified?
Spleen and pancreas
What is the head to abdomen ratio BEFORE 32 weeks?
Head > abdomen
What is the head to abdomen ratio from 32-36 weeks?
Head = abdomen
What is the head to abdomen ratio AFTER 32 weeks?
Head > abdomen
When is the femur length most accurate?
B/w 14 and 22 weeks
Why is it important to assess the femur?
To detect skeletal and chromosomal abnormalities
What femur should you measure?
Measure the femur closest to the transducer
Make femur perpendicular to the sound beam
Describe how you find the femur on the fetus
- Follow the ado down to the iliac bones
- Move the transducer to the anterior aspect of the fetus
- Rotate transducer until femur is elongated
What part of the femur do you measure?
Only the diaphysis
Do you include the epiphysis?
No
When is the epiphysis of the femur seen?
Not until 32 weeks
What are the second trimester additional measurements? Do you have to include them if you have them from the detailed exam?
Cerebellar size Nuchal fold Ventricular size Binocular distance All long bones
Don’t have to do these measurements if you have them from the detail scan
What are the land marks used when measuring the cerebellum?
Cavum septi pellucidi and cerebellum with cistern magna
Can the cerebellar measurement be used in deterring gestational age?
Yes
When and how is the cerebellar used to measure gestational age?
From 14-22 weeks
The # of weeks in mm corresponds to the gestational age
EX: 15mm = 15 weeks
When is the nuchal FOLD thickness measured?
16-20 weeks
How do you measure the nuchal FOLD thickness?
Outer occipital to outer skin surface
What is considered an abnormal value for the nuchal FOLD thickness?
> or = 6mm
What % of fetuses with a thickened nuchal fold will have down syndrome?
45%
Where do you put the calibers when measuring the cistern magna?
From the edge of the cerebellum to inner occipital bone
What is the upper limit of normal when measuring the cistern magna?
1cm
Seeing the cisterna magna rules out what % of the spinal defects and DWM?
> 90%
What structure does the ventricular atrium (Trigone) use to assess?
Hydrocephalus
At what level on the fetus is the trigone measured?
Slightly higher then the BPD
Which vent of the trigone do you measure?
Vent farthest from the probe
What is the upper limit of normal for the trigone?
1cm
What is the trigone or atria?
Area where body, temporal horn and occipital horn meet
When is the binocular distance (orbital) taken?
Detailed scan at 18 weeks
Describe the biocular distance (orbital)
Transverse image through both orbits as well as the cranium
Does the orbital distance change with gestational age?
Yes
Where are the calibers placed when measuring the orbits?
Outer orbit to outer orbit
What can the orbital distance inform use about the fetuses eyes?
Hypotelorism and hypertelorism
What part of the fetuses eyes do you need to see to ensure their liable?
Lens
Are both sides of the long bones scanned?
Yes
Why are the long bones scanned?
To assess skeletal dysplasia
What long bones are scanned first?
Femur and humerus
If the femurs and humeri are abnormal what other bones do you scan?
Tibia, fibula, radius, ulna, clavicles and chest circumference
Why is the foot length measured?
For gestational age
Specifically why are the phalanges assessed?
For down syndrome
What other bones are assessed and indicators for down syndrome?
Nasal bones
Are the hands and feet affected by fibrous dysplasia?
No
How is the biometry plotted on the graph to assess gestational age?
Plotted according to LMP
What are 2 reasons to have an US to assess size?
LGA (large for gestational age) and SGA (small for gestational age)
When is a fetus considered LGA?
> 90%
Why can a fetus be LGA?
Maternal obesity, post dates or gestational diabetes
How does gestational diabetes mellitus affect the fetus?
The abdomen grows at a increased rate bc of increased exposure to glucose
What is macrosomia?
When the fetus weight is > 4000 grams
Why do fetuses with macrosomia have increased morbidity and mortality rates?
Because they are difficult to deliver
What can occur to a macrosomia fetus, whose mother has diabetic?
Fetus can become hypoglycemic
When is the fetus considered SGA?
Biometry < 10%
Why may SGA occur?
May due to intrauterine growth restriction (IUGR)
What can cause IUGR? (8)
Multiple pregnancies Maternal hypertension Placenta insufficiency Vascular dx Anemia, poor nutrition Smoking Substance abuse Chromosomal abnormalities
What are fetuses with SGA at risk for?
Increased risk of fetal morbidity or mortality
What is asymmetric IUGR?
Head normal but abdomen small
What causes asymmetric IUGR?
Due to placenta insufficiency
What % of cases incorporate asymmetric IUGR?
75%
What semester does asymmetric IUGR present itself?
3rd trimester
What is symmetric IUGR?
All measurements are small
What causes symmetric IUGR?
Usually due to chromosomal abnormalities
What % of cases incorporate symmetric IUGR?
25%
What semester does symmetric IUGR present itself?
2nd trimester
