Birth Size Z-Score & Percentile
Z-scores express a measurement as standard deviations from the population mean for that gestational age and sex, enabling direct comparison across GAs and ages. Infants born below the 10th percentile (z < −1.28) are classified as small for gestational age (SGA); those above the 90th (z > +1.28) as large for gestational age (LGA). Reference choice matters: the Fenton 2013 chart uses a smoothed international fetal-infant composite, while the Olsen 2010 chart reflects a contemporary U.S. hospital population. The Chou 2026 NICU chart provides contemporary U.S. birth-size references derived specifically from newborns admitted to neonatal intensive care units. The Chou 2026 nursery chart complements it with references for well, normal-nursery newborns between 35.0 and 41.6 weeks of gestation.
SGA
< 10th
z < −1.28
AGA
10th–90th
−1.28 ≤ z ≤ +1.28
LGA
> 90th
z > +1.28
weeks
days (0–6)
Fenton: 22–42w · Olsen 2010: 23–41w · Chou 2026 NICU: 22–41w · Chou 2026 nursery: 35.0–41.6w
Growth charts to compare
NICU — derived from infants admitted to NICUs; apply only in NICU settings, not for nurseries. Chou 2026 (nursery) is built from well-newborn nursery infants (35.0–41.6 weeks) — the appropriate choice for healthy term/late-preterm newborns.
Fenton TR, Kim JH. A systematic review and meta-analysis to revise the Fenton growth chart for preterm infants. BMC Pediatr. 2013;13:59. doi:10.1186/1471-2431-13-59
Olsen IE, Groveman SA, Lawson ML, Clark RH, Zemel BS. New intrauterine growth curves based on United States data. Pediatrics. 2010;125:e214-24. doi:10.1542/peds.2009-0913
Olsen IE, Lawson ML, Ferguson AN, Cantrell R, Grabich SC, Zemel BS, et al. BMI curves for preterm infants. Pediatrics. 2015;135:e572-81. doi:10.1542/peds.2014-2777
Chou F-S, Yeh H-W, Hsueh C, Olsen IE, Tolia VN, Clark RH. Birth-size reference charts for newborns admitted to the neonatal intensive care units. BMC Pediatr. 2026;26. doi:10.1186/s12887-026-06889-1 (NICU reference)
Chou F-S, Yeh H-W, Hsueh C, Olsen IE, Tolia VN, Clark RH. Birth-size reference charts for normal nursery newborns between 35.0 and 41.6 weeks. BMC Pediatr. 2026. doi:10.1186/s12887-026-07275-7 (well-newborn nursery reference, 35.0–41.6 weeks)
Frequently asked questions
Which reference chart should I use?
Fenton, Olsen, and Chou 2026 (NICU) are intrauterine/NICU-admission references; Chou 2026 (nursery) is built from healthy nursery newborns 35.0–41.6 weeks. Choose the reference that matches your population.
What defines SGA and LGA?
SGA is below the 10th percentile (z < −1.28) and LGA is above the 90th (z > 1.28); the 3rd and 97th percentiles mark the extremes.
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