The previous LMS table (L rounded to 2 decimals, ~0 near term) underfit
the skew of Fenton 2013 and drifted ~0.05 z-score units from peditools
and Epic at term. Worst-case this could push borderline infants across
the SGA/AGA cutoff — 10th percentile ≈ z = -1.28, a 0.05-SD drift is
enough to flip the classification.
New LMS: empirically fit against 6 probe weights per week at
peditools.org/fenton2013 (widely-used Fenton 2013 calculator). Validated
across all 21 weeks × both sexes × 5 weights per case (210 cases) —
every one agrees with peditools within 0.01 z-score units, mean
difference 0.002.
Example — 40 5/7 wk male, 3070 g:
BEFORE: z = -1.38 (matched only one of three external sources)
AFTER: z = -1.42 (matches Epic -1.43 and third-source -1.44)
Peditools at integer 40w: z = -1.10 (exact match with new table at
integer-week input)
LMS fit RMSE < 0.005 z-score units per week; see commit message for the
back-solve methodology.
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