#175 Ultrasound in Pregnancy Flashcards
In regards to ultrasound, does lower frequency provide better penetration or resolution?
Better penetration at the expense of resolution
During early pregnancy, an abdominal transducer with what frequency is typically used? Transvaginal transducer frequency?
Transabdominal 5MHz
Transvaginal 5-10MHz or higher
What is included in a standard obstetric ultrasound exam?
Fetal presentation and number, amniotic fluid volume, cardiac activity, placenta position, fetal biometry, and an anatomic survey. Maternal cervix and adnexa examined as clinically appropriate and when technically feasible
After what gestational age can a fetal anatomy examination be performed?
Approximately 18wks
What are the essential elements of a standard examination of fetal anatomy?
Head, face, and neck: lateral cerebral ventricles, choroid plexus, midline falx, cavum septum pellucidi, cerebellum, cisterna magna, upper lip
Chest: Heart 4 chamber, left and right outflow tracts
Abdomen: stomach (presence, size, and situs), kidneys, bladder, umbilical cord insertion site and vessel number
Spine
Extremities
Fetal sex
What is a limited examination in regards to ultrasound?
Performed when a specific question requires investigation, can be performed in any trimester to estimate AFI, evaluate cervix, assess embryonic or fetal viability, etc.
What is a specialized examination in regards to ultrasound?
AKA detailed, targeted, or 76811 ultrasound. It is more extensive than the standard ultrasound exam. performed if there is an increased risk of an anomaly. Fetal Doppler US, BPP, fetal echo, or additional biometric measurements
Can you use a mean sac diameter to date a pregnancy?
Not recommended
At what crown rump length should you see a heart beat?
`7mm
At what mean gestational sac diameter without embryo can you diagnose failed pregnancy?
25mm
What should you do if a low-lying placenta or placenta previa is suspected early in gestation?
Verification of location in third trimester by repeat ultrasound
When is gestational age by ultrasound most accurate?
First-trimester crown-rump measurement
Starting at what gestational age is fetal biometry performed rather that crown-rump length?
14 weeks
How do you measure a biparietal diameter? What structures are included? What structures are excluded?
Measurement from outer edge of proximal skull to inner edge of distal skull. Measured at the level of the thalamus and cavum septi pellucidi. Cerebrall hemispheres should not be visible.
what is the term for a flattened head shape?
Dolichocephalic
What is the term for a rounded head shape?
Brachycephalic
Does head shape affect reliability of biparietal diameter measurement? of head circumference?
Head circumference may be more reliable at estimating GA as accuracy not affected by head shape
At what gestational age can measurement of femoral diaphysis length be used for dating?
14 weeks
How do you measure femoral shaft length with ultrasound (beam of insonation perpendicular or parallel to shaft?), what should be excluded?
Long axis most accurately measured with beam of insonation perpendicular, excluding the distal femoral epiphysis
How is head circumference measured?
Same plane as biparietal diameter. Around the outer perimeter of the calvarium
How do you measure abdominal circumference, what structures included/excluded?
Measure at skin line on a true transverse view at the level of the umbilical vein, portal sinus, and fetal stomach
What is the error of fetal weight prediction by ultrasound?
Up to 20% in either direction
What is the ALARA principle?
As-low-as-reasonably-achievable principle regarding use of imaging technology including ultrasound assessment of fetus
How should you determine viability with FHR in first trimester?
M-mode scanning or conventional two-dimensional real-time US with video archiving
True or false: Transabdominal probes should be cleaned with a high-level disinfectant between uses?
False. Should be adequately cleansed between patients with soap and water or a disposable disinfectant spray or wipe
What is the sensitivity of ultrasound to diagnose fetal anomalies?
Overall sensitivity ~40%, with a range from less than 15% to higher than 80%
What is ACOG’s stance on ultrasound without medical indication to obtain views of the fetus and obtain a “keepsake” picture or determine fetal sex?
Inappropriate and contrary to responsible medical practice.
At gestational age of 8w6d of less what is method of measurement to confirm gestational age, what discrepancy would support redating?
CRL. More than 5d
At gestational age of 9w0d to 13w6d what is method of measurement to confirm gestational age, what discrepancy would support redating?
CRL. More than 7d
At gestational age of 14w0d to 15w6d what is method of measurement to confirm gestational age, what discrepancy would support redating?
BPD, HC, AC, FL. More than 7d
At gestational age of 16w0d to 21w6d what is method of measurement to confirm gestational age, what discrepancy would support redating?
BPD, HC, AC, FL. More than 10d.
At gestational age of 22w0d to 27w6d what is method of measurement to confirm gestational age, what discrepancy would support redating?
BPD, HC, AC, FL. More than 14d
At gestational age of 28w0d+ what is method of measurement to confirm gestational age, what discrepancy would support redating?
BPD, HC, AC, FL. More than 21d
At what CRL measurement in mm should multiple second-trimester biometric parameters be used for dating?
84mm (corresponding to 14w0d)
Head circumference is single most-predictive parameter of gestational age between what weeks of gestation?
14-22 weeks
What is the single most-predictive parameter of gestational age between 14-22 weeks of gestation?
Head circumference
What single ultrasound measurement is most predictive of gestational age in the third trimester?
Femur length
What is the requirement for an amniotic fluid pocket to be considered measurable?
the width of the pocket must be at least 1 cm
What is the definition of polyhydramnios?
AFI greater than or equal to 24cm or MVP of 8cm or greater
What % of cases of polyhydramnios are idiopathic?
50-60%
Among first-trimester fetuses with increased nuchal translucency, what proportion will have chromosome defects? What % of those with chromosomal defects will have trisomy 21?
1/3, 50% of that 1/3 will be trisomy 21
What signs on first trimester ultrasound are associated with trisomy 21?
Increased NT, absent nasal bone, tricuspid regurg, abnormal ductus venosus waveforms
What can be measured with ultrasound to predict fetal anemia? What is the cutoff value for concern?
Peak systolic velocity in the fetal middle cerebral artery greater than 1.5 MoM for gestational age (sensitvity of ~75%)
How does Doppler test to monitor for anemia perform in fetuses after 34-35 weeks?
High false positive rate
Does maternal race or ethnicity significantly affect fetal growth? Does this affect how we calculate/interpret EFW?
Race/ethnicity does significantly affect fetal growth and adjusting likely decreases misdiagnosis of IUGR and macrosomia. However, unclear whether this would improve outcomes, so customized growth charts are not widely used
At what % discordance between multifetal pregnancies should further evaluation take place for discordant growth?
20%
If US determines EFW <10%tile, what futher US evaluation should be performed?
Amniotic fluid assessment and Doppler blood flow studies of the umbilical artery. Specialized ultrasound exam of fetal anatomy is also recommended if not performed previously
What monitoring should be performed for growth restricted fetus?
Serial US measurements of fetal biometry, amniotic fluid volume, antenatal surveillance with FHR or BPP testing, Doppler flow assessment of umbilical artery. Do not begin testing before GA when delivery would be considered for perinatal benefit
By what % is risk of perinatal death reduced by including umbilical artery Doppler velocimetry to standard antepartum testing in setting of fetal growth restriction?
29%
When is assessment of chorionicity most accurate?
Early in pregnancy, determination by late first trimester or early second trimester is important for counseling
True or false. There is an increased risk of congenital heart defects in fetuses of monochorionic pregnancies?
True, consider fetal echo, especially if anatomy not clearly seen on specialized ultrasound exam
How frequently should US be performed for dichorionic pregnancy?
Serial surveillance q4-6 weeks after 20wks
How frequently should US be performed for monochorionic twins?
q2 wks starting at 16wks
In absence of other specific indications, what is the optimal time for a single US examinaiton?
18-22 wks
