What is the difference between Small for Gestational Age (SGA) and Intrauterine Growth Restriction (IUGR)?

Difference Between SGA and IUGR

SGA (Small for Gestational Age) is a descriptive term for any fetus or newborn with weight below the 10th percentile, while IUGR/FGR (Intrauterine/Fetal Growth Restriction) is a pathological diagnosis indicating a fetus that has failed to reach its growth potential due to underlying disease processes. 1

Key Conceptual Distinction

The Society for Maternal-Fetal Medicine explicitly recommends abandoning the term "intrauterine growth restriction (IUGR)" for prenatal diagnosis in favor of "fetal growth restriction (FGR)," and reserving "small for gestational age (SGA)" exclusively for newborns with birthweight <10th percentile. 1 This nomenclature clarifies that:

• SGA is a statistical definition applied at birth, describing any infant whose birthweight falls below the 10th percentile for gestational age, sex, and population—meaning up to 10% of normal, healthy infants are classified as SGA by definition 2, 3

• FGR/IUGR is a pathological diagnosis indicating growth failure from placental insufficiency, maternal disease, fetal anomalies, or genetic conditions—these fetuses have not achieved their genetic growth potential 3, 4, 5

Diagnostic Criteria That Differentiate Pathological Growth Restriction

Not all SGA fetuses have pathological growth restriction. The following additional findings distinguish FGR from constitutionally small but healthy fetuses:

Severity Markers

• Estimated fetal weight <3rd percentile indicates severe FGR with stillbirth rates up to 2.5%, regardless of other findings 2, 1
• Abnormal umbilical artery Doppler (elevated resistance, absent or reversed end-diastolic flow) confirms placental insufficiency 2
• Abnormal middle cerebral artery or cerebroplacental ratio indicates fetal brain-sparing redistribution 2
• Oligohydramnios suggests chronic placental dysfunction 2

Growth Velocity

• Crossing centiles downward or inadequate interval growth (AC change <5mm over 14 days, or >30% reduction in growth velocity) indicates progressive pathology 2
• Four of six international guidelines (67%) incorporate reduced growth velocity into FGR definitions 2

Clinical Implications of the Distinction

Risk Stratification

• Fetuses below the 10th percentile have 1.5% stillbirth rate (twice that of normally growing fetuses) and increased risk of severe acidosis, low Apgar scores, and NICU admission 1
• Below the 5th percentile, stillbirth rates increase to 2.5% 1
• FGR fetuses with additional pathological markers require intensive surveillance with serial Doppler studies, while constitutionally small SGA fetuses may need only routine monitoring 2

Management Differences

The American College of Obstetricians and Gynecologists provides specific delivery timing based on severity:

• FGR with EFW 3rd-10th percentile and normal Doppler: weekly umbilical artery Doppler, delivery at 38-39 weeks 1
• Absent end-diastolic velocity: Doppler 2-3 times weekly, delivery at 33-34 weeks 1
• Reversed end-diastolic velocity: delivery at 30-32 weeks 1

Postnatal Considerations

• SGA newborns may be constitutionally small and healthy, requiring only standard newborn care 2, 5
• IUGR infants face acute complications including perinatal asphyxia, hypothermia, hypoglycemia, and polycythemia, plus long-term risks of neurodevelopmental handicaps and adult metabolic disease 5, 3
• IUGR requires interdisciplinary follow-up as a continuum of care, while healthy SGA infants do not 3

Common Clinical Pitfall

By definition, 10% of normal fetuses are below the 10th percentile—these are constitutionally small, not pathologically growth-restricted. 2 The critical error is treating all SGA fetuses as pathological or, conversely, missing true FGR by relying solely on percentile cutoffs without assessing Doppler studies, growth velocity, or additional risk factors. 2, 3

Practical Algorithm

When estimated fetal weight is <10th percentile:

1. Confirm accurate dating with first-trimester crown-rump length 1
2. Perform detailed anatomic survey (10-60% of FGR fetuses have anomalies) 2
3. Assess umbilical artery Doppler to differentiate pathological FGR from constitutional SGA 2, 1
4. Evaluate growth velocity with serial measurements 2-4 weeks apart 2
5. Check for severity markers: EFW <3rd percentile, oligohydramnios, abnormal cerebroplacental ratio 2, 1

If Doppler is normal, growth velocity appropriate, and no severity markers present, the fetus is likely constitutionally small (SGA) rather than pathologically growth-restricted (FGR). 2, 3