Ch 1b Biometry Flashcards
What 4 reasons are why we do fetal biometry?
-To plan the mode/way the baby will be delivered along with the date of delivery
-To screen for aneuploidy (chromosome abnormalities)
-Determining dates for termination
-Fetal growth
EDD vs EDC meaning?
-Estimated date of delivery
-Estimated date of confinement
(both mean same thing)
An important estimator of fetal health is what?
Fetal size
What 4 factors affect fetal biometry?
-Ethnic diversity
-Altitude
-Genetics
-Smoking
(remember these are NOT taken into consideration when dating or assessing growth)
When assessing growth, do we take an average of our measurements or take the largest?
Growth: average
Dating: largest
How can we improve our accuracy when assessing growth?
Average multiple measurements (typically 2-3 measurements done)
Measurements should cluster within how many mm of each other?
Within 2-3mm (outliers are discarded/deleted)
What 3 fetal factors can affect measurements?
-PROM (premature rupture of membranes)
-Hydrocephalus
-Maternal diabetes
(can be mitigated/less severe by obtaining at least a BPD, HC, AC + FL)
Why does PROM affect measurements?
B/c it can cause uterine pressure on the fetal head affecting the BPD
Why do we use multiple parameters when scanning a baby?
B/c it minimizes errors + fetal variations
Landmarks for HC?
Thalamus in center, cavum septum pellucidi anteriorly + interhemispheric fissure at midline
How do we measure the BPD?
Outer parietal bone to inner parietal bone
(parietal bones form the lateral walls of skull)
What if we measure the BPD with the fetal face facing anterior or posterior?
May be inaccurate
What is dolicocephaly + brachycephaly?
Dolicocephaly: long + narrow head
Brachycephaly: short + wide head
(these affect our BPD)
Is a BPD or HC more accurate?
HC
How should a proper HC be done?
Measurement should hug the outer bony skull as tightly as possible + avoid including the skin!
Formula for cephalic index?
BPD/FOD x 100
(FOD = frontal occipital distance)
How is FOD measured?
Outer to outer
What is the normal cephalic index?
Less than 80%
What is the BPD + cephalic index like with a dolichocephalic head?
BPD: short
CI: below normal
(long FOD)
What is the BPD, cephalic index + frontal occipital distance like with a brachycephalic head?
BPD: wide
CI: above normal
FOD: short
Landmarks for TRV cerebellum measurement?
CSP, thalamus, falx cerebri + cerebellar hemispheres
Purpose of the TRV cerebellar diameter?
Assess growth
Is the TRV cerebellar diameter dependent or independent of the fetal head shape?
Independent
What does the binocular distance (BOD) measure?
-Distance from outer edge of right + left fetal eyes
(outer orbit to outer orbit (OOD) distance from edges of skeletal orbit)
Is a binocular distance routinely done?
No, often do BOD when looking for hypertelorism (eyes far apart) and hypotelorism (eyes close together)
Abdominal circumference landmarks?
Stomach, umbilical vein + portal vein
Why do we do an abdominal circumference?
B/c it reflects the development of the abdominal organs (such as liver + spleen)
Are abdominal circumferences usually done correctly?
No, easily mismeasured
Why is it important to measure an abdominal circumference correctly?
B/c it factors into fetal weight + health (rather than just an indicator of fetal age)
Do abdominal circumferences include the skin?
Yes! Should be circular in shape + include the skin
Best fetal position to obtain an abdominal circumference?
When baby is on its side
Do the long bones get affected by fetal molding/position in the womb?
No
Do tall parents typically have tall or short babies?
Tall, results in bigger estimations of size (not really seen in 1st trimester tho)
What is the m/c error when imaging the long bones?
Being oblique + missing the ends of the bones
All fetal limbs should be evaluated for what?
Symmetry
Which long bone length is used for dating + growth?
Femur length
How do we correctly measure the femur length?
-Only measure the shaft/diaphysis (don’t include the 2 epiphysis ends)
-Beam should be perpendicular to the shaft
Should we measure the femur closest or farthest from the transducer?
Closest
What tip can we use when looking for the humerus to measure, so we don’t accidentally measure the femur twice?
-The humerus closest to transducer is easiest to find in relation to head/spine
-Look for the bladder or another landmark when measuring the femur
Which bone often gets mistaken for the femur?
Humerus (b/c have similar measurements)
Is it common to measure distal extremities?
-No, except with achondroplasia (short limbed dwarfism)
-Always observe for symmetry
Why would we measure the distal extremities?
-Help resolve conflict in age estimates
In the forearm, is the radius or ulna larger?
Ulna
Location of radius + ulna?
Radius: lateral, side where thumb is
Ulna: medial, side where pinky is (larger)
If the fetal wrist/hand is rotated, will the ulna + radius be crossed or parallel?
Crossed
In the lower leg, is the tibia or fibula larger?
Tibia
Location of the tibia + fibula?
Tibia: medial, side of big toe
Fibula: lateral, side of pinky toe
Why is an estimate of fetal weight important to tell the obstetrician?
To notify them of any developing problems + to assist in subsequent management of the pregnancy
Which 2 measurements must we be extra careful when using for a weight estimate?
-Abdominal diameter
-Abdominal circumference
(there can be significant variation in weight estimates)
How do we measure the fetal foot?
From the tip of the second toe (or longest toe) to the heel
Why do we measure the foot?
Used for femur/foot ratios (b/c short femurs are associated with trisomy 21)
Short femurs are associated with what?
Trisomy 21
Which measurement is easily mismeasured?
Abdominal circumference
