Newborn infants, 22 3/7 to 42 3/7 weeks at birth: weight, length and head circumference at birth. An approximation of the authors' own size-at-birth calculator.
Size at birth from the meta-analysis behind the Fenton 2025 growth chart. The authors describe the two as the same up to about 37 weeks.
This calculator approximates the Fenton 2025 size-at-birth calculator at fentongrowth.ca, which remains the authoritative source. PediTools offers this version as a convenience.
Weight Z-scores beyond ±3 use WHO's restricted LMS method, as in the authors' calculator.