Are small for gestational age (SGA) and premature infants similar scenarios?

Small for Gestational Age (SGA) and Prematurity Are Distinct but Often Overlapping Conditions

No, small for gestational age (SGA) and prematurity are fundamentally different clinical entities that describe separate aspects of neonatal status—SGA refers to size relative to gestational age, while prematurity refers to timing of birth—though they frequently coexist and compound each other's risks. 1

Key Definitions

Small for Gestational Age (SGA):

• Defined as birthweight <10th percentile for gestational age, regardless of when the infant is born 1
• Includes 18-22% of constitutionally small but healthy babies 1
• Can occur at any gestational age (preterm, term, or post-term) 1

Prematurity:

• Birth occurring before 37 weeks of gestation 1
• Defined purely by timing, independent of fetal size 1
• Can include infants who are appropriate for gestational age (AGA), SGA, or large for gestational age (LGA) 2

Fetal Growth Restriction (FGR):

• Refers specifically to a fetus that has failed to reach its biological growth potential due to placental dysfunction 1
• Often used interchangeably with SGA in clinical practice, though technically distinct 1

Critical Clinical Distinctions

When SGA and Prematurity Coexist:

• Prematurity compounds the risk of adverse outcomes in SGA infants significantly 1
• Studies report a 2- to 5-fold increased rate of perinatal death among preterm FGR fetuses compared with term FGR fetuses 1
• The combination creates higher risks for severe acidosis at birth, low 5-minute Apgar scores, and NICU admissions 1

Independent Risk Profiles:

• SGA preterm infants have higher mortality (7.2%) compared to AGA preterm infants (3.5%) 3
• SGA preterm infants show increased rates of bronchopulmonary dysplasia, retinopathy of prematurity, and necrotizing enterocolitis compared to AGA preterm infants 3, 4
• SGA classification at birth more reliably identifies neonates at risk than antenatal suspicion of FGR alone 3

Practical Clinical Implications

Surveillance and Management Differ:

• Early-onset FGR (<32 weeks) requires corticosteroids and magnesium sulfate for neuroprotection if preterm delivery is anticipated 1
• Umbilical artery Doppler studies are universally advised for suspected SGA pregnancies to guide timing of delivery 1
• Delivery timing for FGR with abnormal Doppler varies from 30-34 weeks depending on severity 1

Neonatal Outcomes:

• SGA preterm infants require more prolonged nasal CPAP, supplemental oxygen therapy, and have higher rates of chronic lung disease at 28 days and 36 weeks 4
• Total NICU and hospital days are significantly longer for SGA preterm infants 4
• However, extreme premature SGA infants can achieve appropriate catch-up growth by term equivalence with proper postnatal nutrition 5

Common Clinical Pitfall

The critical error is comparing premature SGA infants to AGA infants of similar birth weight rather than similar gestational age. 6 This comparison obscures the additional risk that poor intrauterine growth adds to prematurity. SGA infants have significantly smaller body dimensions at birth, more nursery complications, and higher incidence of major neurologic problems than their gestational age-matched AGA counterparts 6.

Long-Term Considerations

• SGA babies constitute 28-45% of nonanomalous stillbirths 1
• Both conditions increase risk for neurodevelopmental delay, childhood and adult obesity, and metabolic disease 1
• FGR is associated with metabolic programming that increases future risk of metabolic syndrome and cardiovascular disease 1