C7.0: 2nd and 3rd Trim. Biometry Flashcards
when is a nuchal lucency done? wks
11-14 wks
how do you measure the nuchal translucency?
inner to inner (echogenic area thru the hypo echoic fluid to echogenic skin)
what is a common mistake when measuring a nuchal lucency?
mistaking the amniotic membrane for the nuchal
do you zoom when doing a nuchal lucency?
yes
what does an increased nuchal lucency indicate?
chromosomal abnormalities
cardiac abnormalities
blood disorders
skeletal dysplasia
what nuchal lucency measurement is normal
~3mm
what are the routine measurements for 2nd and 3rd trimester?
BPD occipital frontal diameter (OFD) cephalic index (CI) head circumference (AC) abdominal circumference (AC) femur length fetal weight
when do you do a BPD?
starting at 12 wks until term
when would you use a BPD for dating of preg? how accurate was it?
in the 2nd trimester if dating wasn’t done in the 1st trimester
+/- 7 days in early 2nd trimester
what intracranial landmarks are used for the BPD measurement and where are they located in the brain?
falx cerebri (anterior and posterior)
thalami (middle)
cavum septi pellucidi (anteriorly)
how should the falx cerebri appear on the US screen?
create a flat line across the screen
How do you measure the BPD?
outer to inner edge of the skull bones across the widest part of the head
when demonstrating the CSP on US, how should it appear and why?
like a box, not an equal sign… showing the box rules out many fetal abnormalities
when imaging the CSP, what landmarks may you see anteriorly?
the anterior horns of the ventricles
which three head measurements are taken at the same level?
head circumference, BPD and occipital frontal diameter
should the HC be around the outside or inside of the skull?
outside
can the OFD and BPD be done at the same time?
yes
how do you measure the OFD?
from the outer margin of the frontal bone to the outer margin of the occipital bone.
the BPD and OFD are used to calculate what?
the cephalic index
how do you calculate cephalic index?
(BPD/OFD) X 100
what are the two things that head shape influences?
BPD and estimation of fetal age
whats the norm range for cephalic index?
75-85%
what is dolichocephaly and brachycephaly?
doli: narrow head
brachy: wide head
when is a fetus considered to have dolichocephaly and brachycephaly?
doli: <75%
brachy: >85%
what can cause dolichocephaly?
if the baby is breached or has low fluid
what 2 measurements can be used to calculate HC? and whats the formula?
BPD and OFD
HC= (BPD + OFD) X 1.57
how is the BPD measured for the HC calculation?
outer to outer, ONLY for this calculation
Is HC more or less dependent on shape than BPD?
less
whats the formula for Abdominal circumference and where are the callipers placed?
AC= (AP diameter of abdo + TRANS diameter of abdo) X 1.57
callipers are placed around the outer surface of the skin
for which measurements do we use an ellipse?
AC and HC
what are the landmarks for where you should take the AC?
umbilical vein draining into portal vein (hockey stick) stomach adrenal glands round shape ribs 3 ossification centres of the spine
what does IUGR stand for?
intrauterine growth restriction
whats the most important measurement to determine fetal growth?
AC
how should you change your scanning window if youre on the fetal abdomen and you are showing the umbilical vein all the way to the skins surface?
you are too inferior on the anterior aspect of the abdomen
how should you change your scanning window if youre on the fetal abdomen and you are showing the kidneys?
you are too inferior on the posterior aspect of the abdomen
which structures should be seen when scanning the fetal abdomen? and which are less frequently seen?
live, kidney, GB, vasculature, bladder
spleen
pancreas
list the head to abdomen ratios before 32 wks, from 32-36 wks and after 36 wks
before 32: head > abdomen
32-36 wks: head = abdomen
after 36 wks: head < abdomen
when is femur length the most accurate?
between 14 and 22 weeks
why is it important to measure the femur?
what is considered an abnormal femur?
assess for skeletal and chromosomal abnormalities
short for gestation age is abnormal
which femur should you measure and what is the most optimal window to measure the femur?
which part of the femur do you measure?
the femur closest to the probe
make the femur perpendicular to the sound beam
only the diaphysis (blunt ends)
how do you find the femur?
follow the abdomen down to the iliac bones, move the prove to the anterior aspect of the fetus… rotate until the femur is elongated
when are the epiphysis seen?
after 32 weeks
list the additional 2nd trimester measurements at are only done once @ the detailed scan
cerebellum size nuchal fold cisterna magna size ventricular size binocular distance all long bones
what is the nuchal fold measurement? how and when do we do it?
thickness of the skin at the back of the neck
measure from outer occipital bone to outer skin surface
from 16-20 wks
what is an abnormal nuchal fold and what % of fetuses with an abnormal measurement will have Down syndrome?
> or equal to 6mm is abnormal
45%
how do you measure the cisterna magna and what is the upper limit of normal?
from edge of the cerebellum to inner occipital bone
upper of normal is 10mm
seeing the cisterna magna rules out what % of spinal defects and DWM?
> 90%
how will the cerebellum and cistern mag appear with spinal bifida?
banana shaped…. and you wont see the cisterna magna
why do we measure the ventricular atrium/trigone?
and where in the brain to we take this measurement compared to the BPD?
to assess for hydrocephalus (dilation of ventricles dur to blockage
slightly more cephalic than the BPD
which ventricle do we measure and where?
ventricle furthest from the probe at the parietal occipital fissure
whats the upper limit of normal for the trigone?
10mm
what should the anterior horns measure at under 24 wks
<20mm
what is the trigone/atria?
where the body, temporal horn and occipital horn meet
How many ‘soft’ markers are concerning when doing a fetal assessment? why
2+
they increase the chances that baby has chromosomal abnormalities
The yolk sac can often be seen on an EV ultrasound when the MSD is how many mm?
The embryo can often be seen on an EV ultrasound when the MSD measures how many mm?
8mm
16mm
The yolk is readily visualized on a transabdominal ultrasound when the MSD measures how many mm?
The embryo is readily visualized on a transabdominal ultrasound when the MSD measures how many mm?
20mm
25mm
when do you use MSD measurement?
4-8wks
what is the mean sac diameter of the gest. sac for 6, 7 and 8 weeks?
how much does it grow every day?
6: 15 mm
7: 20 mm
8: 30 mm
(L+ W+D/3)
1.1 mm
when is CRL used? wks
6-13wks
when is oligohydraminios used? and how do we find it?
with what value is low fluid suspected?
5.5-9 wks
MSD-CRL…. if this value is <5mm
when is the CSP seen on US?
18-27 wks
if youre showing the equal sign instead of the box for the CSP whats the structure youre actually seeking?
columns of the fornix
a normal CSP should be how many times longer than wide?
1.5x
can the cerebellum measurement be used to determine gestation age? if yes, when?
yes
from 14-22 wks, the size in mm will correspond to gestational age
what are the landmarks for where you should take the cerebellar measurement?
cavum septum pellucid and cerebellum with cisterna magna
what plane do we use to image the orbits when measuring the orbital distance? how do we measure them?
does orbital distance change with gestation?
trans plane through the orbits and the skull.
measure outer to outer orbit
yes
what is hypotelorism and hypertelorism?
hypo: eyes too close together
hyper: eyes to far apart
which long bones do we always measure in the detailed scan?
when would you measure the others?
femur and humerus
only if they look abnormal… looking for skeletal dysplasia
are clavicles considered a long bone?
yes
why would we do a chest circumference?
sm chest is lethal because lungs cant develop properly
are feet and hands effects by skeletal abnormalities?
no
can you measure foot length for gestation age?
yes
which two boney features can be an indication of Down syndrome?
polydactyl
and an absent or hypoplasic nasal bone
b/w what growth percentile in a fetus considered normal?
10-90%
whats considered large for GA?
what can it be caused by?
> 90%
caused by gestation diabetes or maternal obesity
what happens in terms of fetal abdomonal growth if mom has GDM?
abdomen grows faster due to more exposure to glucose
whats macrosomia? what are potential complications for baby?
weight >4000g
- increased morbidity and mortality b/c they’re hard to deliver
- baby can become hypoglycaemic post delivery
whats considered SGA? what causes it?
what are potential complications for baby?
<10%
can be due to IUGR which can be caused by multiple pregancies, placental insufficiency, chromosomal abnormalities
increased morbidity and mortality
compare symmetrical to asymmetrical IUGR
symmetrical:
- all measurements are sm
- commonly caused by choromosomal abnormalities
- account for 25% of all IUGR
- occurs in early 2nd trimester
asymmetrical:
- head is norm size but abdomen is small
- commonly caused by placental insufficiency
- 75% of all IUGR
- occurs in late 3rd trimester
