Third Trimester Growth Scan: Protocol, Measurements, and Interpretation

Third trimester growth scans are ordered to monitor fetal size, detect growth restriction or macrosomia, assess amniotic fluid, and evaluate fetal wellbeing. This is not a full anatomy survey — it's a targeted exam with specific measurements and clinical goals.

Indications

• Suspected intrauterine growth restriction (IUGR / FGR)
• Maternal diabetes (risk of macrosomia)
• Hypertension or preeclampsia
• Multiple gestation
• Prior growth-restricted pregnancy
• Decreased fetal movement
• Oligohydramnios or polyhydramnios
• Post-dates pregnancy (≥ 41 weeks)
• Interval growth assessment after abnormal earlier scan

Biometric Measurements

These four measurements are used to calculate estimated fetal weight (EFW):

1. Biparietal Diameter (BPD)

• Axial plane through the widest part of the skull
• Landmarks: Cavum septum pellucidum (CSP) anteriorly, thalami centrally, absent cerebellum (it should not be visible in the correct plane)
• Measure from outer edge of near skull to inner edge of far skull (outer-inner)
• If the skull is dolichocephalic (elongated) or brachycephalic (round), BPD may be inaccurate — use head circumference instead

2. Head Circumference (HC)

• Same plane as BPD
• Trace the outer perimeter of the skull or use the ellipse tool
• More reliable than BPD alone when head shape is abnormal

3. Abdominal Circumference (AC)

• The single most sensitive measurement for detecting growth restriction and macrosomia
• Axial plane through the fetal abdomen at the level of the stomach and portal sinus (junction of the left and right portal veins — appears as a short linear echogenic structure)
• Circular, not oval — if the section looks oval, you're off-axis
• The spine should be visible but the kidneys should NOT be (too low if kidneys visible)
• Trace the outer skin edge

4. Femur Length (FL)

• Longest bone in the body — use the ossified diaphysis only
• Both ends should be flat, not angled (if angled, you're measuring obliquely)
• Don't include the femoral head or distal epiphysis

Estimated Fetal Weight (EFW)

EFW is calculated from the biometric measurements using published formulas. The Hadlock formula (BPD + HC + AC + FL) is most commonly used.

Reporting EFW:

• Give the EFW in grams
• Express as a percentile for gestational age — this is more clinically meaningful than weight alone
• Reference the curve used (Hadlock, INTERGROWTH-21st, WHO)

Growth restriction thresholds:

• EFW < 10th percentile: Small for gestational age (SGA)
• EFW < 3rd percentile: Severe FGR
• AC < 5th percentile: Often the earliest and most sensitive indicator

Macrosomia threshold: EFW > 4,000–4,500 g (institution-dependent)

Amniotic Fluid Assessment

Amniotic fluid volume is evaluated by one of two methods:

Amniotic Fluid Index (AFI)

• Divide the uterus into four quadrants (umbilicus = horizontal divider, linea nigra = vertical)
• Measure the deepest vertical pocket of fluid in each quadrant — no fetal parts, no umbilical cord in the measurement
• Add all four measurements
• Normal AFI: 8–24 cm (some use 5–25 cm)
• Oligohydramnios: AFI < 5 cm
• Polyhydramnios: AFI > 24–25 cm

Single Deepest Pocket (SDP)

• Identify the largest vertical pocket of clear amniotic fluid
• Must be at least 1 cm in horizontal width
• Normal SDP: 2–8 cm
• Oligohydramnios: SDP < 2 cm
• Polyhydramnios: SDP > 8 cm

Which to use: SDP is increasingly preferred in singleton pregnancies (less operator variability, better predictive value). AFI is still commonly used and may be institutionally required. For twin gestations, use SDP for each sac.

Fetal Presentation

Document fetal presentation:

• Cephalic (head down): vertex, face, brow
• Breech: frank (legs extended), complete (flexed), footling (feet presenting)
• Transverse / oblique

In the third trimester, presentation guides delivery planning. External cephalic version (ECV) may be offered for breech presentation after 36 weeks.

Placenta and Cervix

• Confirm placental location and reassess lower placental edge distance from internal os if previously low-lying
• Cervical length is not routine in third trimester unless specifically ordered
• Assess for placental maturity (Grannum grade — optional)

Umbilical Artery Doppler

Umbilical artery Doppler is indicated when FGR is suspected or confirmed. It reflects placental resistance.

Measure: S/D ratio, RI, and waveform pattern

Finding	Significance
Normal	Forward diastolic flow, S/D ratio appropriate for GA
Elevated S/D (RI > 0.70)	Increased placental resistance
Absent end-diastolic flow (AEDF)	Severe placental compromise
Reversed end-diastolic flow (REDF)	Critical — associated with fetal acidemia

AEDF and REDF are findings that warrant immediate clinical escalation.

Middle Cerebral Artery (MCA) Doppler

MCA PSV is used to screen for fetal anemia (not for growth assessment). A PSV > 1.5 MoM (multiples of the median) indicates possible fetal anemia.

In FGR: brain-sparing physiology may show low MCA RI (cerebral vasodilation) even before umbilical artery changes — the cerebroplacental ratio (CPR = MCA RI ÷ umbilical artery RI) < 1.0 suggests brain-sparing and adverse outcome risk.

Reporting Template

Document:

1. Biometric measurements and gestational age estimated from each measurement
2. EFW in grams with percentile
3. Amniotic fluid (AFI or SDP, with interpretation)
4. Fetal presentation
5. Placental location
6. Umbilical artery Doppler (if performed)
7. Interval change from prior study (if available)

Reporting language:

• "Fetal biometry: BPD 82 mm (33w2d), HC 295 mm (33w0d), AC 285 mm (32w4d), FL 63 mm (32w6d). EFW 2,080 g (48th percentile for 33 weeks). Amniotic fluid: AFI 14 cm, normal. Cephalic presentation. Posterior placenta, not low-lying."
• "AC measures at the 4th percentile for gestational age. EFW 1,610 g (6th percentile at 32 weeks). Umbilical artery Doppler: S/D ratio 3.8 with preserved diastolic flow. Findings consistent with early fetal growth restriction."

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SonoBuddy's Calculators include EFW (Hadlock), AFI interpretation, and gestational age from biometry. OB Measurements covers normal biometric percentiles.