Overview
This lecture covers common mistakes to avoid in second and third trimester fetal ultrasound, focusing on accurate measurements and correct identification of fetal anatomy.
Fetal Measurements: Best Practices
- Always use optimized images and correct planes for head, abdominal, and femur measurements.
- Small measurement errors are more significant at growth extremes (<10th or >90th percentile).
- For BPD (biparietal diameter), ensure the head is symmetric; place cursors leading edge to leading edge.
- OFD (occipitofrontal diameter) should be measured middle to middle, with an intact calvarium and no orbits/posterior fossa in view.
- Abdominal diameter must be taken at the liver level, at the stomach bubble and interhepatic umbilical vein, skin edge to skin edge.
- Avoid oblique planes, compressed abdomens, or misidentifying ribs and fat as edges in abdominal measurements.
- Ensure all abdominal diameter measurements are within 10 mm for reliability.
- Femur length: keep bone axis parallel to transducer, measure only diaphysis/metaphysis, not the epiphysis.
Common Measurement Errors: Case Insights
- Discard technically poor measurements; do not average with good ones.
- Inaccurate measurements can lead to incorrect fetal weight estimates and inappropriate clinical decisions.
Fetal Anatomy Imaging: Mistakes & Solutions
Fetal Head
- Do not mislabel normal anatomical variants (e.g., cavum vergae, cavum velum interpositum) as pathology.
- Correctly measure lateral ventricles: use a level, axial plane, calipers at inner ventricular lining.
- Use smallest technically accurate measurement for ventricles; oblique images artificially increase size.
- Choroid plexus cysts must be discrete, round, >2 mm, and seen in all planes—smudgy cysts are normal "spongy choroid."
Posterior Fossa
- Dandy-Walker variant is often overcalled in fetuses <18 weeks or with oblique images; confirm in axial plane after 18 weeks.
- Mega cisterna magna should be assessed with overall brain anatomy; isolated cases have good prognosis.
Face & Orbits
- Evaluate both nose/lips (for clefts) and both orbits for a "normal" face.
- False positives for clefts can occur; confirm with additional or 3D images.
- Identify both bony orbits; absence may indicate severe facial malformations.
Kidneys
- Adrenal glands are often mistaken for kidneys—assess location (stomach above kidneys) and echogenicity.
- Search lower abdomen/pelvis for ectopic or pelvic kidneys if one is missing.
Heart
- Dropout at ventricular septum may mimic VSD (ventricular septal defect); change angle and use color Doppler to confirm.
- Four-chamber view alone is insufficient—image aorta and pulmonary outflow tracts in the correct plane.
- True pericardial effusion >2 mm; avoid confusing hypoechoic myocardium with fluid.
- Correct imaging plane shows the continuity of interventricular septum and aortic wall to exclude overriding aorta.
Other Pitfalls
- Ascites must be confirmed in multiple locations; a hypoechoic rim alone may be pseudoascites (musculature).
Key Terms & Definitions
- BPD (Biparietal Diameter) — measurement across fetal head at widest part.
- OFD (Occipitofrontal Diameter) — measurement from front to back of fetal skull.
- Diaphysis — shaft of a long bone.
- Epiphysis — end part of a long bone, not included in fetal femur measurement.
- Macrosoomia — excessively large fetus.
- Choroid Plexus Cyst — fluid-filled space in the brain, often benign and transient.
- Dandy-Walker Variant — malformation involving cerebellar vermis and fourth ventricle.
- VSD (Ventricular Septal Defect) — hole in the heart’s ventricular septum.
- Mega Cisterna Magna — enlarged cisterna magna space in the brain.
- Pseudoascites — hypoechoic rim mimicking ascites, actually abdominal muscle.
Action Items / Next Steps
- Review measurement techniques for BPD, OFD, abdominal diameter, and femur to ensure accuracy.
- Practice distinguishing anatomical variants from pathology in fetal ultrasounds.
- Complete any assigned readings on fetal ultrasound measurement protocols.