Methodology and Calculation Standards

WHO and CDC reference calculations are kept separate. The birthday handoff is explicit. Growth Velocity is date-based change arithmetic, Corrected Age is a timeline calculation, and Mid-Parental Height is a rough family-height estimate.

Child Growth Lab primarily serves people using common CDC and AAP practice in the United States. WHO Child Growth Standards are used from birth through the day before the second birthday; CDC growth references are used from the second birthday onward. That handoff can also change recumbent length to standing height and weight-for-length to BMI-for-age, so a value can shift when the standard and measurement method change together.

Fenton, Olsen, and other preterm charts are not selected automatically by the Baby calculator. Growth Velocity, Corrected Age, and Mid-Parental Height are separate arithmetic or family-background tools, not growth-percentile standards. Using these methods does not imply endorsement by CDC, WHO, AAP, or any other organization.

• Strict date-only arithmetic and calendar-birthday eligibility where dates are used.
• Exact unit conversion, no premature rounding, and explicit submission.
• Immutable submitted snapshots separate inputs from the displayed result.
• Extreme-value flags request a measurement recheck; they are not diagnoses.
• One result is not a trend.
• Parent-facing results are separated from technical details.

Calendar birthdays determine eligibility from the second birthday through the day before the 20th birthday. Exact elapsed days are converted to age months using days ÷ 30.4375. Production stature parameters come from the chart-specific statage.csv; production weight parameters come from wtage.csv. L, M, and S are linearly interpolated between adjacent official anchors, with no nearest-row fallback or extrapolation.

The CDCref_d.csv/SAS program supports independent non-endpoint verification and does not override formal chart-specific endpoints. CDC modified-z BIV thresholds are used only to prompt measurement review and are not diagnoses. Formula version: cdc-lms-v2.

The Baby calculator accepts any one or more of weight, recumbent length, and head circumference. Weight-for-length appears only when weight and length are both submitted. Age-based indicators use exact integer age in days from actual birth and measurement dates, from birth through the day before the second birthday; corrected age is not substituted automatically.

Weight-for-length supports the official 45.0–110.0 cm recumbent-length source range. Between adjacent 0.1 cm length keys, the calculator first linearly interpolates L, M, and S at the submitted length, then calculates one WHO adjusted z-score from those interpolated parameters; it does not interpolate final z-scores. For weight-for-age and weight-for-length beyond ±3 z, the WHO extension rule is applied. Before age 2, the CDC WHO SAS method adds 0.7 cm to standing height to approximate recumbent length, but this page accepts recumbent length and does not automatically perform that conversion. Extreme-value flags only prompt measurement review; they are not diagnoses.

Formula version: who-igrowup-v2. WHO data is used under Permission Request 202609680. Child Growth Lab is independent, is not affiliated with or endorsed by WHO, uses no WHO logo, and does not offer restricted originals as a standalone download.

Eligibility runs from the second birthday through the day before the 20th birthday. Exact age months use elapsed days ÷ 30.4375, and CDC BMI L, M, and S values are linearly interpolated between adjacent anchors. BMI is kg/m² or 703 × lb/in².

For BMI at or below P95, including exact P95, the final CDC result uses CDC 2000 LMS. Only BMI greater than P95 uses CDC 2022 Extended BMI. This formula boundary is distinct from chart-use guidance: the 2000 chart is usually used at or below P95, the Extended chart is recommended above P97, and either chart may be used from P95 through P97. The original LMS z, final CDC z, and modified BMI z are distinct values. BIV uses only modified BMI z with strict thresholds below −4 or above +8. Severe obesity is BMI at least 120% of P95 or BMI at least 35 kg/m². All categories are screening references, not diagnoses.

Production data is normalized from official bmi-age-2022.csv; retained bmiagerev.csv is not the current production path. Formula version: cdc-kids-bmi-v3.

The tool calculates arithmetic change from complete dated height/length pairs, weight pairs, or both. A height/length pair must record the same measurement method on both dates: standing height or recumbent length. The calculator does not automatically apply the CDC SAS 0.8 cm method conversion. It uses exact elapsed days, 365.25 days/year, and 30.4375 days/average month. The short height/length interval prompt is based on complete calendar months. It does not implement a velocity percentile or diagnostic threshold. Version: growth-velocity-v3.

The default mode uses the original estimated due date; recorded gestational age at birth is an alternative mode. Recorded GA must be 20+0 through 36+6, or the due-date adjustment must be 22–140 days. The 20+0 lower limit is a technical data-entry guard, not a survival, prognosis, resuscitation, treatment, or medical-decision boundary. Future birth dates are rejected; future assessment or target dates are allowed. Results distinguish before, at, and after the 40-week arithmetic reference and never present a negative parent-facing corrected age.

Dates use date-only YYYY-MM-DD integer-day arithmetic without time of day or timezone and are not intended for hour-level inpatient or treatment timing. Corrected-age use depends on purpose. NICE recommends using corrected age through 2 years when assessing functional and developmental skills. Current AAP growth guidance states that corrected age is used for growth assessment in infants and children born preterm until age 3. These are different purposes: the term-infant WHO birth-to-2 chart range is not a universal corrected-age cutoff for children born preterm. The applicable chart, assessment, and clinical team determine use. This calculator does not automatically select a growth chart or decide whether correction should continue, and it does not change a birthday, legal age, or vaccination schedule; CDC vaccination guidance generally uses chronological age for preterm infants, subject to its stated exceptions. Version: corrected-age-v3.

All units convert to canonical centimeters before the historical sex adjustment: boys add 13 cm before averaging; girls subtract 13 cm before averaging. This is equivalent to ±6.5 cm from the parent average. The approximate family-height range is ±10.16 cm / ±4 in. Parent heights must be 50–274.32 cm after conversion; this is a technical entry guard, not a normal adult-height range. Accepted results must have a positive center and range.

The method uses reliable adult-height records after growth is complete and does not correct for parental age, reconstruct younger peak height, apply regression to the mean, or implement the retained 2024 study's experimental model. The result is not a guarantee or individualized prediction interval, and RCPCH regression/z-score calculation is not implemented. Version: tanner-mid-parental-v3.

Original source files, checksums, source URLs, and licensing records are preserved where applicable. Source permission does not imply endorsement. Public descriptions do not transfer rights in CDC, WHO, professional-society, or published research materials.

Independent scripts do not import the corresponding production calculation function: validate-cdc-exact-age.mjs, validate-who-baby.mjs, validate-kids-bmi.mjs, validate-growth-velocity.mjs, validate-corrected-age.mjs, and validate-mid-parental-height.mjs. They check appropriate golden vectors, boundaries, hashes, unit equivalence, and other contracts.

Unit tests, Playwright, axe, lint, TypeScript, build, metadata, publishing checks, and Preview acceptance remain release gates. These are engineering and editorial controls, not medical certification or clinical validation.

Calculations retain precision until display formatting. Percentiles, categories, ranges, and flags are references rather than diagnoses. Calculator inputs remain browser-local. See Privacy and the Medical Disclaimer.

A verified source, calculation, content, accessibility, or delivery correction receives relevant regression coverage and updates the internal route metadata and sitemap lastModified value when appropriate. Formatting-only changes do not justify a false review date.