Small for Gestational Age (SGA): Definition and Management
SGA is defined as a fetus or newborn with estimated fetal weight (EFW) or abdominal circumference (AC) below the 10th centile for gestational age, while fetal growth restriction (FGR) represents pathological restriction of genetic growth potential requiring evidence of placental dysfunction or abnormal Doppler studies. 1
Key Definitions and Distinctions
SGA vs. FGR - Critical Difference:
- SGA: Size measurement below 10th centile at a single time point, may be constitutionally small and healthy 1
- FGR: Pathological growth restriction with evidence of compromise, including:
- EFW or AC <3rd centile, OR
- EFW or AC <10th centile PLUS abnormal uterine artery Doppler (mean PI >95th centile) and/or abnormal umbilical artery Doppler (PI >95th centile) 1
Severe SGA: Below the 3rd centile 2
Important: Not all SGA fetuses are growth restricted - many are constitutionally small with normal outcomes. The distinction matters because management differs significantly 3
Antenatal Management of SGA Fetus
Initial Assessment
When SGA is identified:
- Verify gestational age using first-trimester crown-rump length (11-14 weeks) 2
- Perform detailed ultrasound to exclude:
Surveillance Protocol
For SGA WITHOUT FGR features (normal Doppler, normal amniotic fluid, normal movements):
- Ultrasound surveillance every 2 weeks 1
- Umbilical artery Doppler is the primary surveillance tool - proven to reduce perinatal morbidity and mortality in high-risk pregnancies 1
- Monitor amniotic fluid using single deepest vertical pocket (SDVP <2 cm indicates oligohydramnios) - but do not use in isolation for management decisions 1
- Serial growth assessments at minimum 2-3 week intervals 2
Watch for progression to FGR:
- Suboptimal growth: AC/EFW crossing centiles >2 quartiles (equivalent to 50 centiles) at ≥32 weeks 1
- Static growth: No forward growth velocity in EFW or AC measured at least 14 days apart 1
- Development of abnormal Doppler studies
Timing of Delivery
For SGA fetuses (without FGR features):
- Delivery at 39+0 to 39+6 weeks is recommended 1
- This is based on the DIGITAT trial showing no additional risk from planned birth at this gestation compared to expectant management
- Do NOT routinely deliver at 37-38 weeks - early term delivery increases neonatal unit admissions and potential long-term adverse effects (special educational needs) without improving outcomes in low-risk SGA 1
Risk stratification approach improves outcomes:
- Low-risk SGA managed expectantly beyond 37 weeks shows lower neonatal composite adverse outcomes (9% vs 22%) and lower NICU admissions (13% vs 39%) compared to routine early delivery 5
For FGR (with placental dysfunction):
- Timing depends on severity and Doppler findings
- Refer to Level IIb or III maternity ward if EFW <1500g or delivery anticipated before 32-34 weeks 2
- Absent or reversed end-diastolic flow in umbilical artery indicates severe early-onset FGR requiring earlier delivery 1
Antenatal Interventions
Corticosteroids: Recommended if delivery anticipated before 34 weeks 1
Magnesium sulfate: Should be prescribed for preterm deliveries before 32-33 weeks 1
NOT recommended:
- PDE5 inhibitors - insufficient evidence 1
- LMWH for treatment/prevention of SGA/FGR (unless indicated for thromboembolic disease risk) 1
Mode of Delivery
- Cesarean section is NOT systematically recommended for FGR 2
- For vaginal delivery: continuous fetal heart rate monitoring during labor with lower threshold for intervention 2
- Regional anesthesia preferred 2
Neonatal Management of SGA Infant
Immediate Care
Initial management priorities:
- Combat hypothermia - maintain heat chain with survival blanket 2
- Ventilation support with pressure-controlled insufflator if needed 2
- Close monitoring of capillary blood glucose - SGA infants at high risk of hypoglycemia 2
Risk Profile
SGA newborns face increased risks:
- Mortality: 2-4 times higher than non-SGA infants at same gestational age 2
- Stillbirth rate: 1.5% for weights <10th centile; 2.5% for weights <5th centile 4
- Severe acidosis at birth, low 5-minute Apgar scores, NICU admissions 4
- Preterm SGA have worst outcomes: 2-5 fold increased perinatal death rate compared to term SGA 4
Important distinction: SGA by Fenton postnatal growth charts more reliably identifies neonates at risk of adverse outcomes than antenatal suspicion of FGR alone 6
Long-term Follow-up
Most SGA children (majority) experience spontaneous catch-up growth completed by age 2 years 7
For SGA children who remain short:
- Treatment with recombinant human growth hormone is effective for increasing adult height 7
Metabolic risks with rapid catch-up growth: SGA infants with rapid catch-up and marked weight gain are at increased risk for:
- Premature adrenarche
- Early puberty
- Polycystic ovary syndrome (girls)
- Insulin resistance and obesity
- Type 2 diabetes and metabolic syndrome in adulthood 7
Neurodevelopmental considerations:
- SGA can affect multiple areas of neurodevelopment at different life stages
- Better outcomes in SGA infants with spontaneous catch-up growth 7
- Higher risk of minor cognitive deficits and school problems 2
Essential Follow-up
Due to potential long-term risks, adequate characterization at birth is imperative to allow:
- Appropriate follow-up protocols
- Early detection of metabolic abnormalities
- Monitoring of growth trajectory
- Neurodevelopmental surveillance 7
Common Pitfalls to Avoid
Do not equate all SGA with FGR - this leads to unnecessary interventions in constitutionally small but healthy fetuses 3
Do not deliver all SGA fetuses at 37 weeks - this outdated practice increases neonatal morbidity without benefit in low-risk cases 1
Do not use amniotic fluid assessment alone to determine management - it has poor predictive value in isolation 1
Do not abandon the terms "symmetrical" or "asymmetrical" FGR - these are no longer prognostic and should not guide management 1
Do not use low birthweight (<2500g) as a diagnostic criterion - this outdated term doesn't account for gestational age 1