Gestational Age Assessment

A newborn weighs 2,800 g — is that small for gestational age or appropriate? You can't classify without first determining gestational age, and the physical exam tells you things the due date can't.

Gestational age assessment determines a newborn's maturity independent of maternal dates, which can be inaccurate. The Ballard (New Ballard) Score is the standard tool, combining six neuromuscular maturity criteria (posture, square window, arm recoil, popliteal angle, scarf sign, heel to ear) and six physical maturity criteria (skin texture, lanugo, plantar creases, breast tissue, ear cartilage, genitalia). Scores range from -10 to 50, corresponding to 20–44 weeks gestation. The assessment is most accurate when performed within the first 12–48 hours of life; after 96 hours, neuromuscular findings change as the infant adapts to extrauterine tone. Once gestational age is established, the newborn is plotted on a growth curve and classified: small for gestational age (SGA, below 10th percentile), appropriate for gestational age (AGA, 10th–90th percentile), or large for gestational age (LGA, above 90th percentile). This classification drives clinical decision-making — SGA newborns are at high risk for hypoglycemia, polycythemia, and temperature instability; LGA newborns are at risk for birth injury and hypoglycemia. Preterm is less than 37 weeks, term is 37–41 6/7 weeks, and post-term is 42 weeks or greater.

Key Distinctions

Don't confuse gestational age (weeks of maturity) with birth weight classification (SGA/AGA/LGA) — a preterm infant can be LGA and a term infant can be SGA. Students mix up neuromuscular signs: increased tone and flexion indicate greater maturity, not distress. The Ballard Score is not the same as APGAR — APGAR evaluates transition to extrauterine life at 1 and 5 minutes, while Ballard evaluates developmental maturity within the first 48 hours.

Clinical Pearl

More creases, more cartilage, more flexion — more mature. A floppy infant with smooth soles and thin translucent skin is telling you they arrived early.