Second and Third Trimester Fetal Anatomy, Biometry, and Anomalies
Standard planes
Biparietal diameter and head circumference use an axial plane at the level of the thalami and cavum septi pellucidi, with a thin symmetric calvarial rim and the falx interrupted anteriorly. Abdominal circumference is measured where the stomach bubble and portal sinus/umbilical vein are seen, on a circular section with symmetric ribs. Femur length uses only the ossified diaphysis, perpendicular to the beam, excluding distal epiphyses. Each measurement must be reproducible; a single erroneous AC can shift estimated fetal weight by hundreds of grams.
Anatomic survey
The cisterna magna measures 2–10 mm, the lateral ventricular atrium under 10 mm, and the nuchal fold under 6 mm at 15–22 weeks. Assess the four-chamber heart, outflow tracts, three-vessel trachea view, spine in three planes, both kidneys, bladder, abdominal wall, cord insertion, and genitalia. Soft markers - echogenic intracardiac focus, mild pyelectasis, choroid plexus cyst, echogenic bowel - are usually isolated and benign but are interpreted with prior risk.
Traps
Ventriculomegaly must be measured at the atrium, inner-to-inner, perpendicular to the ventricular axis; measuring at the frontal horn overestimates. Head circumference is distorted by molding and by anomalously shaped skulls, so BPD and HC disagree in dolichocephaly. BPD loses dating accuracy after about 24 weeks, when AC becomes the key growth parameter. Do not call an encephalocele before confirming the defect is a calvarial defect with herniated content, and do not confuse physiologic bowel echogenicity with pathologic.
Rule of thumb
Normal values scale with gestation: cisterna magna 2–10 mm and atrium under 10 mm at any gestational age.