How is Small for Gestational Age (SGA) differentiated from Intrauterine Growth Restriction (IUGR)?

Differentiating SGA from IUGR Using Ultrasound with Doppler

Yes, ultrasound with Doppler velocimetry is the primary modality for distinguishing pathological intrauterine growth restriction (IUGR/FGR) from constitutionally small for gestational age (SGA) fetuses, with umbilical artery Doppler being the key discriminating tool. 1

Core Distinction

The fundamental difference is that SGA is a statistical definition (estimated fetal weight or abdominal circumference <10th percentile), while IUGR/FGR represents a pathological process where the fetus has failed to reach its growth potential due to placental insufficiency or other pathology. 1

• SGA fetuses with normal Doppler studies most likely represent constitutionally small, not pathologically growth-restricted fetuses 2
• IUGR is defined as growth below the fetus's in-utero growth potential, not just below population norms 3, 4

Diagnostic Algorithm Using Ultrasound with Doppler

Step 1: Confirm Size Abnormality

• Verify estimated fetal weight or abdominal circumference is <10th percentile using appropriate growth curves 1
• Serial measurements showing decreasing percentile growth suggest pathological restriction requiring intensive monitoring 5

Step 2: Umbilical Artery Doppler Assessment (Primary Discriminator)

Umbilical artery Doppler is the most critical tool for differentiating pathological FGR from constitutional SGA, as it identifies placental insufficiency and hypoxic growth-restricted fetuses. 1

• Normal umbilical artery Doppler in a small fetus = likely constitutional SGA, not pathological IUGR 2
• Abnormal umbilical artery Doppler findings indicating FGR include: 1, 6
  • Elevated resistance/pulsatility index
  • Absent end-diastolic flow (AEDV)
  • Reversed end-diastolic flow (REDV)

Small for gestational age fetuses with normal Doppler studies showed no increased morbidity compared to average-sized fetuses, while those with abnormal Doppler had significantly increased cesarean sections for fetal distress, NICU admissions, and neonatal morbidity. 2

Step 3: Additional Doppler Parameters for Risk Stratification

Once IUGR is suspected based on abnormal umbilical artery Doppler, additional Doppler studies help assess severity:

Middle Cerebral Artery (MCA) Doppler:

• Decreased MCA pulsatility index indicates cerebral vasodilation ("brain-sparing effect") in response to hypoxemia 6
• Abnormal cerebroplacental ratio combined with abnormal uterine artery Doppler and EFW >3rd percentile discriminates SGA pregnancies at risk for adverse outcomes 5
• SGA fetuses with abnormal MCA Doppler are associated with neurodevelopmental problems at follow-up 5

Maternal Uterine Artery Doppler:

• Abnormal uterine artery Doppler with abnormal cerebroplacental ratio helps discriminate SGA pregnancies at risk for adverse outcomes at delivery 5
• Particularly useful when FGR is associated with pre-eclampsia 5
• Uterine artery pulsatility index among FGR fetuses diagnosed at 20-28 weeks inversely correlates with birth weight 5

Ductus Venosus Doppler:

• Represents advanced fetal compromise when abnormal 6
• Reversed A-wave flow is associated with neonatal demise 5
• Abnormalities reflect increased central venous pressure and cardiac compromise 6

Clinical Implications Based on Doppler Findings

The distinction matters critically because management differs dramatically:

For Constitutional SGA (Normal Doppler):

• May require only standard newborn care 1
• Still has increased risk of stillbirth, so cannot be completely ignored 1

For Pathological IUGR/FGR (Abnormal Doppler):

• Requires intensive surveillance with umbilical artery Doppler, biophysical profile or nonstress testing 1
• Earlier delivery timing based on Doppler severity: 1
  • 30-32 weeks for reversed end-diastolic flow
  • 33-34 weeks for absent end-diastolic flow
  • 38-39 weeks for normal Doppler but confirmed FGR
• Abnormal biophysical profile is a strong argument for delivery, though gestational age must be considered 5

Critical Pitfalls

Umbilical artery Doppler is a better predictor of neonatal outcome than estimated fetal weight alone. 2 Do not rely solely on size measurements without Doppler assessment when evaluating small fetuses.

Before 34 weeks' gestation, stillbirths among FGR fetuses followed worsening umbilical artery and ductus venosus Doppler findings plus abnormal biophysical profile, though precise management directives remain unclear for these preterm gestations. 5

Absent or reversed end-diastolic flow in the umbilical artery is independently associated with perinatal mortality, developmental disorders, and delay among FGR fetuses 5