General Sonography and Maternal Fetal Health
General Sonography and Maternal Fetal Health
This domain tests image optimization, safety, and obstetric measurement: dating, biometry, amniotic fluid, and umbilical artery Doppler.
Core rules
- ALARA: use the lowest output that yields diagnostic images. Monitor thermal index (TIB when bone is in the beam) and mechanical index; limit dwell time, especially in the first trimester and with Doppler.
- Match probe and preset to depth. Higher frequencies improve resolution in thin patients and early gestation; harmonics improve contrast but can change texture.
- First-trimester dating uses crown-rump length (CRL) up to 13+6 weeks and ≤84 mm; CRL is the most accurate single dating parameter. Later, use BPD/HC, then AC/FL.
- Biometry planes: BPD at the thalami; HC outer-to-outer; AC at the portal sinus and stomach; FL measures the diaphysis, not the epiphyses.
- Amniotic fluid: AFI ≤5 cm is oligohydramnios and ≥24 cm is polyhydramnios; single deepest pocket ≤2 cm and ≥8 cm.
- Umbilical artery Doppler: free loop, angle near 0°, fetal quiescence; absent or reversed end-diastolic flow is high risk.
Worked example
At 32 weeks the AC is at the 5th percentile. AC is the most growth-sensitive measurement, so first verify the plane at the portal sinus, place calipers on skin, and avoid probe compression. Then calculate EFW and check umbilical artery Doppler and fluid; do not repeat AC alone.
Traps
Measuring AC too high (liver dome) or too low; dating by BPD in the second trimester; calling oligohydramnios from AFI without confirming a pocket; ignoring Doppler angle; over-gaining until subtle fluid disappears.