What is the difference between Small for Gestational Age (SGA) and Fetal Growth Restriction (FGR) in a pregnant individual with a history of complications or risk factors, such as hypertension or diabetes?

Distinguishing SGA from FGR: A Critical Clinical Differentiation

Small for Gestational Age (SGA) is a statistical definition (estimated fetal weight <10th percentile), while Fetal Growth Restriction (FGR) represents a pathological process where the fetus has failed to reach its growth potential due to placental insufficiency—the key distinction lies in using umbilical artery Doppler to identify which small fetuses are truly at risk. 1, 2

Core Definitions

SGA is purely a size-based classification:

• Estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th percentile for gestational age 1
• Includes 18-22% of fetuses who are constitutionally small but healthy 1
• Does not necessarily indicate pathology 1

FGR represents pathological growth failure:

• A fetus that has not reached its biological growth potential due to placental dysfunction 1
• Associated with significant perinatal morbidity, mortality, and long-term neurodevelopmental problems 1
• Requires intensive surveillance and earlier delivery 1, 2

The Diagnostic Algorithm: Moving from SGA to FGR

Step 1: Confirm Size Abnormality

• Verify EFW or AC is <10th percentile using appropriate population-based growth curves 1, 2
• Accurate first-trimester dating (crown-rump length 11-14 weeks) is essential—misdating is a common pitfall 1, 3
• Severe cases with EFW <3rd percentile have stillbirth rates up to 2.5% and should be considered pathological FGR regardless of other findings 1, 4

Step 2: Apply Doppler Assessment to Differentiate

Umbilical artery Doppler is the primary tool to distinguish pathological FGR from constitutional SGA 2, 5, 3:

Findings indicating pathological FGR:

• Elevated umbilical artery resistance (elevated pulsatility index) 2, 3
• Absent end-diastolic flow 1, 2, 6
• Reversed end-diastolic flow 1, 2, 6
• Abnormal middle cerebral artery Doppler (brain-sparing) 1, 2
• Abnormal cerebroplacental ratio combined with abnormal uterine artery Doppler 1, 2

Additional severity markers:

• Oligohydramnios (suggests chronic placental dysfunction) 1, 4
• Decreasing percentile growth on serial measurements (crossing centiles downward) 1, 2, 4
• Abnormal ductus venosus Doppler with reversed A-wave (associated with neonatal demise) 1, 2

Step 3: Risk Stratification in Patients with Hypertension or Diabetes

For pregnant individuals with hypertension:

• History of maternal hypertension or vascular disease are established risk factors for FGR 1
• Maternal uterine artery Doppler pulsatility index inversely correlates with birth weight in FGR diagnosed at 20-28 weeks 1, 2
• Abnormal uterine artery Doppler combined with abnormal cerebroplacental ratio and EFW >3rd percentile discriminates SGA pregnancies at risk for adverse outcomes 1, 2

For pregnant individuals with diabetes:

• While diabetes typically causes macrosomia, concurrent vascular disease can lead to FGR 1
• The same Doppler-based algorithm applies to differentiate pathological FGR from SGA 2, 5

Clinical Implications: Why the Distinction Matters

Constitutional SGA (Normal Doppler):

• May require only standard newborn care 2
• Still carries 1.5% stillbirth rate (twice normal) 1, 4
• Weekly umbilical artery Doppler surveillance recommended 6
• Delivery at 38-39 weeks if EFW 3rd-10th percentile with normal Doppler 4, 6

Pathological FGR (Abnormal Doppler):

Surveillance intensity escalates based on Doppler severity 6:

• Decreased end-diastolic flow: Weekly Doppler, deliver at 37 weeks 6
• Absent end-diastolic flow: Doppler 2-3 times weekly, deliver at 33-34 weeks 1, 6
• Reversed end-diastolic flow: Hospitalization, corticosteroids, cardiotocography 1-2 times daily, deliver at 30-32 weeks 1, 6

Outcomes Data Supporting This Approach:

• Absent or reversed end-diastolic flow is independently associated with perinatal mortality, developmental disorders, and delay 1, 2
• Before 34 weeks, stillbirths in FGR follow worsening umbilical artery and ductus venosus Doppler plus abnormal biophysical profile 1, 2
• SGA fetuses with abnormal middle cerebral artery Doppler show neurodevelopmental problems at follow-up 1, 2

Critical Pitfalls to Avoid

Do not rely on fetal heart rate monitoring alone:

• Normal heart rate patterns do not exclude FGR and should never be the sole surveillance method 6
• Heart rate changes occur late in deterioration—after significant Doppler abnormalities are already present 6
• 75-95% of FGR pregnancies with absent/reversed end-diastolic flow require cesarean delivery for intrapartum heart rate abnormalities, even when antepartum testing was reassuring 6

Do not use birthweight curves for preterm gestations:

• Birthweight curves are inappropriate for identifying SGA at early gestational ages due to disorders associated with preterm delivery 3
• In-utero curves represent physiological growth more reliably 3
• SGA classification by postnatal Fenton charts (approximately equivalent to <3rd percentile on in-utero charts) more reliably identifies neonates at risk than antenatal suspicion of FGR 7

Do not treat all small fetuses identically:

• Given the difficulty differentiating constitutional SGA from pathological FGR, it is typical for all fetuses with EFW <10th percentile to be treated comparably in clinical practice 1
• However, Doppler assessment allows more precise risk stratification and avoids unnecessary interventions in constitutionally small fetuses 2, 5, 3

Management Framework for High-Risk Patients

For patients with hypertension or diabetes and a small fetus:

1. Confirm accurate dating (first-trimester crown-rump length) 1, 3
2. Perform umbilical artery Doppler as the primary diagnostic tool 2, 5, 3
3. If Doppler normal: Weekly Doppler surveillance, deliver 38-39 weeks 4, 6
4. If Doppler abnormal: Escalate surveillance and delivery timing based on severity (see above) 6
5. Consider additional Doppler parameters (middle cerebral artery, uterine artery, ductus venosus) for further risk stratification 1, 2
6. Serial growth assessment every 3-4 weeks (not less than 2 weeks) to detect crossing centiles 1, 4, 6
7. Antenatal corticosteroids if delivery anticipated <34 weeks 1, 6
8. Magnesium sulfate for neuroprotection if delivery <32 weeks 1, 6