Differentiating IUGR from SGA
The key distinction is that SGA is purely a size definition (estimated fetal weight <10th percentile), while IUGR/FGR represents a pathological process where the fetus has failed to reach its growth potential and is at risk for adverse outcomes—differentiation relies primarily on umbilical artery Doppler studies to identify placental insufficiency. 1, 2
Core Definitions
SGA (Small for Gestational Age):
- Defined as estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age 1, 2
- This is a statistical definition only—many SGA fetuses are constitutionally small and healthy 2, 3
- Can be applied both prenatally and postnatally (birth weight <10th percentile) 2, 3
IUGR/FGR (Intrauterine/Fetal Growth Restriction):
- An SGA fetus that has not reached its growth potential and is at risk for adverse sequelae 1
- Represents pathological growth failure, not constitutional smallness 4, 5
- The Society for Maternal-Fetal Medicine recommends using "FGR" for prenatal diagnosis and reserving "SGA" for newborns 2
Diagnostic Approach to Differentiation
Step 1: Confirm Size Abnormality
- Verify EFW or AC is <10th percentile using appropriate growth curves 1, 2
- Ensure accurate pregnancy dating using first-trimester crown-rump length 2, 3
- Severe cases with EFW <3rd percentile should be considered FGR regardless of other findings, with stillbirth rates up to 2.5% 2, 3
Step 2: Umbilical Artery Doppler Assessment (The Critical Differentiator)
- This is the primary tool to distinguish pathological FGR from constitutional SGA 1
- Umbilical artery Doppler can differentiate the hypoxic growth-restricted fetus from the nonhypoxic small fetus, reducing perinatal mortality and unnecessary interventions 1
Doppler findings indicating FGR:
- Elevated umbilical artery resistance 2
- Absent end-diastolic flow 1, 2
- Reversed end-diastolic flow 1, 2
- Abnormal middle cerebral artery or cerebroplacental ratio (brain-sparing) 2
Normal Doppler suggests constitutional SGA 1
Step 3: Additional Markers of Pathological FGR
Growth velocity abnormalities:
- Crossing centiles downward on serial measurements 2
- AC change <5mm over 14 days 2
30% reduction in growth velocity 2
- Serial measurements should be at least 2-3 weeks apart 1, 3
Associated findings suggesting FGR:
- Oligohydramnios (chronic placental dysfunction) 2
- Structural anomalies (10% of FGR fetuses have congenital anomalies) 1
- Abnormal biophysical profile 1
Step 4: Consider Timing and Etiology
Early-onset FGR (<32-34 weeks):
- More likely chromosomal anomalies, syndromes, viral infections 1, 6
- Symmetric growth restriction more common 1
- Requires detailed fetal structural survey 1
Late-onset FGR (≥32-34 weeks):
- Placental insufficiency predominates (25-30% of all FGR) 6
- Associated with hypertension and maternal vascular disease 1, 6
- Asymmetric FGR with "head-sparing" more common 1, 6
Clinical Implications of the Distinction
FGR fetuses require:
- Intensive surveillance with umbilical artery Doppler (weekly to 2-3 times per week depending on severity) 1, 2
- Biophysical profile or nonstress testing twice weekly 1
- Earlier delivery based on Doppler findings (30-32 weeks for reversed flow, 33-34 weeks for absent flow, 38-39 weeks for normal Doppler) 2
- Antenatal corticosteroids if delivery anticipated before 34 weeks 3
Constitutional SGA fetuses:
- May require only standard newborn care if truly constitutional 2
- Still have approximately 1.5% stillbirth rate (twice normal), so some surveillance warranted 2
Common Pitfalls
- Using size criteria alone without Doppler assessment—this misses the critical distinction between pathological and constitutional smallness 1
- Inaccurate pregnancy dating—this is essential for proper diagnosis and must be based on first-trimester measurements 2, 3
- Failing to perform serial measurements—single measurements cannot identify growth deceleration 2, 3
- Using birth weight curves for preterm fetuses—in utero curves represent physiological growth more reliably 3