Module 7 : 2nd and 3rd Trimester Biometery Flashcards
1st trimester measurements
- mean sac diameter (MSD)
- crown rump length (CRL)
- nuchal lucency
mean sac diameter
- used when embryo is not identified
- ensure you see the double decidual sign
- used from 4-7 or 8 weeks
- length+width+height/3
crown rump length
- used between 6-13 weeks
- most accurate measurement to predict gestational age =/- 3 days
- tip of head to end of rump
- in a neutral position
oligohydramnios measurement
- 5 1/2 - 9 weeks
- MDS(mm) - CRL(mm)
< 5mm = early oligo
nuchal lucency
- done between 11-14 weeks to rule out chromosomal abnormalities
- magnify to see nuchal lucency well
- DO NOT MISTAKE AMNIOTIC MEMBRANE FOR NUCHAL LUCENCY
- measure on to on
indications for nuchal lucency
- increased nuchal
- chromosomal abnormalities
- cardiac abnormalitites
- blood disorders
- skeletal dysplasia
upper limit of normal for nuchal
- 3 mm is upper limit of normal
2nd and 3rd trimester measurements
- Biparietal diameter (BPD)
- head circumference (HC)
- abdominal circumference (AC)
- femur length
- femur weight
biparietal diameter BPD
- most accurate measurement in the 2nd trimester for dating of pregnancy
- +/- 7 days in early 2nd trimester
- falx should create a line flat across the screen
inter cranial landmarks for BPD
- falx cerebri
+ anterior and posteriorly demonstrated in the brain - thalami in the middle of the brain
- caved septum pellucidi anteriorly landmark in the brain
calliper placement for BPD
- leading edge to leading edge
where to measure BPD
- at the widest part across the head
- outer skull to inner skull
normal cavum septum pellicidum CSP
- 1.5x longer than wider
head circumference (HC)
- BPD only measured outer edge to outer edge in this calc only
- along the falx
- don’t do BPD on same picture
- BPD and OFD can be used to calculate head circumference
- all machines have ellipse
- HC less dependant on shape than BPD
occipital frontal diameter (OFD)
- can be done in conjunction with the BPD
- taken at same level as BPD
- measure from OUTER margin of frontal bone to OUTER margin of occipital bone
- BPD and OFD used to calculate CEPHALIC INDEX
cephalic index
- head shape influences BPD estimation of of fetal age
cephalic index equation
CI = BPD / OFD x 100
normal cephalic index range
75% - 85%
dolichocephalic range
< 75%
- narrow head
brachycephalic range
> 85%
- wide head
head circumference equation
HC = (BPD + OFD) x 1.57
abdominal circumference
- callipers placed around outer surface of skin
abdominal circumference equation
AC = AP diameter = TRV diameter x 1.57
AC landmarks
- umbilical vein draining into portal sinus
- stomach
- adrenal veins
- round in shape
- ribs
- 3 ossification centres of spine
fetal abdominal structures identified on US
- liver
- kidneys
- gallbladder
- vasculature
- urinary bladder
- sometimes spleen , pancreas
head to abdomen ratios before 32 weeks
- head > abdomen
head to abdomen ratios 32-36 weeks
- head = abdomen
head to abdomen ratios after 36 weeks
- head < abdomen
femur length FL
- most accurate between 14-22 weeks
- important measurement to assess for skeletal abnormalities
+ short for gestational age is abnormal
how to measure femur length
- measure femur closest to the transducer
- make femur perpendicular to sound beam
finding femur
- follow abdomen down to iliac bones
- move transducer to the anterior aspect of fetus
- rotate transducer until femur is elongated
where to measure femur length
- only measure diaphysis (shaft)
- do not include epiphysis
+ not seen until > 32 weeks
2nd trimester additional measurements
- cerebellum size
- nuchal fold
- cisterna magna
- ventricular size
- binocular distance
- all long bones
- only done once at detailed scan
cerebellar measurement
- landmarks = caves septum pellucidi and cerebellum with cisterna magna
- can be used for gestational age
- from 14-22 the # in mm corresponds to gestational age
nuchal fold thickness
- 16-20 weeks
- measure from outer occipital bone to outer skin surface
- > /= 6mm abnormal
- 45% of Down syndrome will have thickened nuchal fold
cisterna magna
- measure from edge of cerebellum to inner occipital bone
- upper limit of normal is 1cm or 10mm
- seeing cisterna magna rules out >90% of spinal defects in DWM
+ spina bifida
ventricular atrium (trigone)
- to assess for hydrocephalus (ventricles dilated because of CSF)
- taken slightly more cephalic than BPD
- measure ventricle farthest from the transducer
- measure at the parietal occipital fissure
upper limit of normal for ventricular trigone
- 1cm - 10mm
anterior horns at 24 weeks
- should be less than 2 cm
10 10 6
- lateral ventricles posterior
- cisterna magna
- nuchal fold
soft markers
- individuals can be born
- won’t be harmful to baby
biocular distance (orbital)
- transverse image through both orbits as well as cranium
- orbital distance changes with gestation
- outer orbit to outer orbit
- hypotelorism and hypertelorism
+ hyper to far
+ hypo to far
all long bones
- have to do right and left
- to asses skeletal dysplasia
- femur
- radius and ulna
- humerus
- clavicles
- chest circumference
other measurement s
- foot length for gestational age
- phalanges for down syndrome
- nasal bones for Down syndrome
gestational size compared to dates
- biometry plotted on graphs to assess for gestational size
- plotted according to LMP to an early ultrasound
- LGA and SGA reasons to assess fetal size
under 10th percentile
- not good
- not in safe environment
- very small
over 90th percentile
- sometimes gestational diabetes or genetics
- very large
large for gestational age LGA
- due to maternal obesity or gestational diabetes
+ GDM = abdomen grows at increased rate due to increased exposure to glucose
macrosomia
- weight >4000 grams
- have increased morbidity and mortality because they are difficult to deliver
- baby can become hypoglycaemic post delivery
small for gestational age SGA
- may be due to intrauterine growth restriction IUGR
- IUGR due to
+ multiple pregnancy
+ placental insufficiency
+ chromosomal abnormalities - increased risk of fetal morbidity or mortality
asymmetric IUGR
- head normal size but abdomen small (shunting oxygen to brain)
- usually due to a placenta insufficiency
- 75% of all IUGR
- occurs in 3rd trimester
symmetric IUGR
- all measurements small
- usually due to chromosomal abnormality
- 25% of all IUGR
- occurs in 2nd trimester
