Fetal Biometry and Growth
Key Takeaways
- BPD, HC, AC, and FL are standard second/third-trimester biometry; composite EFW uses Hadlock or similar formulas.
- HC measured on transtentorial plane at thalamic level; BPD from outer-to-inner or outer-to-outer per protocol.
- AC is single ellipse at junction of umbilical vein and portal sinus on true transverse abdomen.
- Growth restriction suspected when EFW <10th percentile or AC lagging with abnormal Doppler—not single parameter alone.
- Biometry every 3–4 weeks is typical for growth surveillance; more frequent if pathologic Doppler.
Quick Answer: Measure BPD/HC at thalamic level, AC at UV-portal sinus junction, FL full length. EFW <10th %ile or serial AC fall suggests growth restriction. Always state GA source. Wrong plane = wrong percentile.
Why Biometry Dominates the RDMS Blueprint
Second- and third-trimester obstetrics is the largest RDMS OB/GYN domain (~37%), and fetal biometry sits at its center. Registry items rarely ask you to recite formulas—they show still images with calipers and ask whether the measurement is valid, what growth pattern is suggested, or what Doppler component comes next. Success requires plane discipline plus interpretation of percentiles and trends.
Standard Biometric Parameters
| Parameter | Correct Plane | Common Pitfall |
|---|---|---|
| BPD | Transtentorial axial at thalamic level; CSP visible; no cerebellum | Oblique skull including mandible widens BPD |
| HC | Same plane; ellipse around outer skull table per protocol | Ellipse on cerebellar plane |
| AC | True transverse abdomen: stomach left, spine posterior, UV at portal sinus | Oblique slice through kidney inflates AC |
| FL | Longest femoral diaphysis, exclude epiphysis | Shortened if shadowed or partial length |
BPD and HC Technique
Most departments measure BPD outer skull table to inner table; some use outer-to-outer—consistency on serial scans matters more than debating formulas on the exam. HC is traced as an ellipse at the same level as BPD. If the cerebellum is visible, the plane is too posterior for BPD/HC—registry images test this constantly.
AC Technique (Most Sensitive for Asymmetric IUGR)
On a true transverse abdomen:
- Stomach bubble left of midline (normal situs)
- Spine and descending aorta posterior
- Umbilical vein entering liver at portal sinus
- Single ellipse at skin line (not liver margin)
AC errors disproportionately shift estimated fetal weight (EFW) because abdominal soft tissue reflects nutritional status.
Femur Length
Measure perpendicular to the shaft. If shadowed by maternal bone or fetal position, wait for fetal movement or adjust angle—do not accept a foreshortened diaphysis. Isolated short FL may prompt skeletal survey and genetics counseling per protocol.
Estimated Fetal Weight and Growth Patterns
Composite formulas (Hadlock HC-AC-FL ± BPD) yield grams and percentile for gestational age.
| Pattern | Biometry Clues | Likely Mechanism |
|---|---|---|
| EFW <10th %ile | All parameters low or AC lagging | SGA—constitutional vs pathologic |
| Asymmetric IUGR | HC normal/spared, AC <10th %ile | Placental insufficiency (late) |
| Symmetric growth restriction | All parameters low proportionally | Early insult, infection, aneuploidy |
| Macrosomia/LGA | EFW >90th %ile, often elevated AC | Diabetes, constitutional large size |
Single biometry never diagnoses IUGR without serial trend, amniotic fluid assessment, and umbilical artery Doppler when indicated.
Worked Scenario: Asymmetric Growth Restriction
32 weeks: HC 30th percentile, AC 5th percentile, UA Doppler shows elevated S/D ratio. Registry asks next study component: umbilical artery + middle cerebral artery Doppler with cerebroplacental ratio context. Calling "constitutionally small" without Doppler is incorrect.
Gestational Age Assignment
After ~14 weeks, dating uses early CRL or second-trimester composite biometry. If LMP and ultrasound differ beyond institutional tolerance, re-date from earliest reliable scan. Late third-trimester single BPD is unreliable for dating—registry trap: re-dating at 34 weeks from one BPD measurement.
| Timing | Best Dating Source |
|---|---|
| ≤13+6 weeks | CRL |
| 14–26 weeks | Composite biometry |
| Third trimester | Only if no prior dating; wide error |
Serial Growth and Surveillance Intervals
ACOG/AIUM typically recommend growth scans every 3–4 weeks when surveillance is indicated (hypertension, diabetes, prior IUGR, decreased fetal movement). Diagnosing true growth restriction requires plateau or decline across serial measurements—not one point at the 8th percentile with normal Doppler in a constitutionally small fetus.
Growth velocity: Document change in EFW or AC percentile between studies. A drop from 40th to 8th percentile over three weeks warrants explicit reporting and follow-up interval.
Macrosomia and Diabetic Surveillance
EFW >90th percentile or clinical concern for macrosomia (>4000–4500 g delivery risk context) influences delivery planning. Sonographic EFW carries ~15% error—report as estimate with percentile, not exact weight. In diabetic pregnancy, AC often rises first in fetal overgrowth.
Doppler Integration (Growth Restriction Workup)
| UA Doppler Finding | Significance |
|---|---|
| Elevated S/D, PI, RI | Placental resistance increased |
| Absent end-diastolic flow | Severe insufficiency |
| Reversed end-diastolic flow | Critical—immediate MFM context |
Middle cerebral artery brain-sparing (low PI) with abnormal UA supports asymmetric IUGR. Registry vignettes link biometry to Doppler next steps.
Customized vs Population Charts
Some programs customize growth to maternal height, weight, ethnicity, and parity. ARDMS stems typically assume standard population percentiles unless explicitly stated otherwise.
Exam Traps
- HC measured on cerebellar/transcerebellar plane
- AC through kidney-level oblique slice
- Calling IUGR on one AC <10th %ile without trend or Doppler
- Re-dating from late third-trimester BPD alone
- Reporting EFW without percentile or GA source
Biometry is arithmetic plus plane discipline. On every image question, name the plane before judging the calipers—wrong level invalidates the percentile and the diagnosis that follows.
Head Circumference and Microcephaly Screening
HC percentiles track cerebral growth independently of abdominal soft tissue. Microcephaly is suspected when HC trends below the 3rd percentile across serial scans or falls more than two quartiles—not from one borderline measurement. Registry images may show normal AC with falling HC percentile, prompting genetics and TORCH correlation. Macrocephaly similarly requires ventricular assessment to exclude hydrocephalus before labeling familial large head.
Humerus and Additional Long Bones
When FL is short, measure humerus length to classify shortening pattern. Rhizomelic shortening (proximal bones affected) suggests skeletal dysplasia rather than placental insufficiency. Document whether shortening is symmetric or isolated.
The abdominal circumference should be measured on a transverse plane that includes:
Asymmetric intrauterine growth restriction is most often suggested when:
After the first trimester, the most accurate biometric parameter for establishing gestational age when early CRL is unavailable is:
A biparietal diameter measurement obtained in an oblique plane that includes the mandible will tend to: