Preterm, Term, Post-Term, SGA, and LGA Newborn Classifications

Newborns are classified by gestational age and by birth weight relative to gestational age. These categories drive risk assessment and nursing priorities in the immediate newborn period. This note reviews the definitions, characteristics, and high-yield risks for preterm, term, post-term, SGA, and LGA infants.

Definitions

• Term infant — born between 37 and 42 weeks of gestation; normal range for a healthy newborn.
• Preterm infant — born before 37 weeks; immature organ systems and higher risk for complications.
• Post-term infant — born after 42 weeks; at risk for meconium aspiration, hypoglycemia, and birth trauma.
• Small for gestational age (SGA) — birth weight below the 10th percentile for gestational age. SGA infants may be preterm, term, or post-term.
• Large for gestational age (LGA) — birth weight above the 90th percentile for gestational age.

Characteristics by Category

• Term: 37–42 weeks; 2,500–4,000 g (5.5–8.8 lbs); minimal risk, routine newborn care.
• Preterm: <37 weeks; <2,500 g (5.5 lbs); risks include respiratory distress, hypoglycemia, hyperbilirubinemia, hypothermia, NEC, and IVH.
• Post-term: >42 weeks; weight variable (often large); risks include meconium aspiration, hypoglycemia, birth trauma, and polycythemia.
• SGA: any gestational age; weight <10th percentile; risks include hypoglycemia, hypothermia, and polycythemia.
• LGA: any gestational age; weight >90th percentile; risks include birth trauma, hypoglycemia, and respiratory distress.

Preterm Infant (Before 37 Weeks)

• Respiratory distress syndrome (RDS) — caused by surfactant deficiency and immature lungs.
• Hypoglycemia — limited glycogen stores and immature gluconeogenesis.
• Hyperbilirubinemia — immature liver function and decreased bowel motility.
• Hypothermia — large surface area to body weight ratio and thin skin.
• Necrotizing enterocolitis (NEC) — serious GI condition more common in preterm infants.
• Intraventricular hemorrhage (IVH) — bleeding into the brain ventricles.

Post-Term Infant (After 42 Weeks)

• Meconium aspiration syndrome — infant passes meconium in utero and aspirates it, causing respiratory distress.
• Hypoglycemia — increased metabolic demand and depletion of glycogen stores.
• Birth trauma — more common due to larger size and reduced amniotic fluid; shoulder dystocia may occur.
• Polycythemia — chronic in-utero hypoxia leads to increased red blood cell mass.
• Dysmaturity syndrome — dry, peeling skin; long nails; decreased subcutaneous fat.

SGA Infant (Below 10th Percentile)

• Definition: birth weight below the 10th percentile for gestational age. Causes include placental insufficiency, maternal malnutrition, or genetic factors.
• Hypoglycemia — limited glycogen stores and increased metabolic demand.
• Hypothermia — decreased subcutaneous fat and large surface area.
• Polycythemia — chronic hypoxia.
• Meconium aspiration — more common; in-utero hypoxia triggers meconium passage.

LGA Infant (Above 90th Percentile)

• Definition: birth weight above the 90th percentile for gestational age; often caused by maternal diabetes.
• Birth trauma — larger size increases risk; shoulder dystocia may occur.
• Hypoglycemia — due to hyperinsulinism, especially in infants of diabetic mothers.
• Respiratory distress — may occur from delayed lung maturation.

Exam Traps

• Do not confuse SGA with preterm — SGA is based on weight, preterm is based on gestational age.
• Do not assume post-term infants are healthy; they are at risk for meconium aspiration and birth trauma.
• LGA infants of diabetic mothers are at high risk for hypoglycemia from hyperinsulinism.
• Preterm infants are at risk for neurodevelopmental delay even after discharge.
• Don't forget preterm infants are at risk for NEC and IVH.
• Watch for polycythemia in both SGA and post-term infants.

Key takeaways

• Gestational age defines preterm/term/post-term; birth weight percentile defines SGA/AGA/LGA — these are independent classifications.
• Preterm infants are most at risk for RDS (surfactant deficiency), hypoglycemia, hypothermia, hyperbilirubinemia, NEC, and IVH.
• Post-term infants risk meconium aspiration, hypoglycemia, birth trauma, polycythemia, and dysmaturity syndrome.
• SGA infants share hypoglycemia, hypothermia, and polycythemia risks regardless of gestational age.
• LGA infants — often from diabetic mothers — risk shoulder dystocia, birth trauma, and hypoglycemia from hyperinsulinism.