What this page is for
Most parent-facing pages answer a practical question: how to calculate corrected age, when to stop correcting, whether vaccines use chronological age, or when to worry. This page does something different. It explains how the site itself is built, what sources were chosen, what has been left out on purpose, and what assumptions are used when the calculator gives an answer.
If you want the plain-English arithmetic first, go to the worked corrected-age examples page. If you want the site's implementation details and editorial rules, stay here.
The arithmetic, exactly as implemented
- All internal calculation is in whole days. Term is defined as 280 days (40+0).
- Gestational age at birth (days) = weeks × 7 + days.
- Chronological age (days) = visit date − date of birth, using UTC-normalised dates so daylight saving cannot shift a result by a day.
- Prematurity (days) = max(0, 280 − gestational age in days).
- Corrected age (days) = chronological age − prematurity. It is negative before the original due date, and is displayed that way rather than clamped to zero.
- Postmenstrual age (days) = gestational age at birth + chronological age.
- Display conversion uses 30.4375 days per month. Weeks and days are shown below 12 weeks, where rounding to months would be misleading.
Why 280 days and UTC-normalised dates are explicit choices
Many corrected-age explanations quietly introduce avoidable error. The two most common are treating term as “nine months” and letting browser timezone behaviour shift a date math result by one day. This site makes both choices explicit.
- Term is 280 days, not “9 months”. Forty weeks × 7 days is the same 40-week basis used in the AAP’s corrected-age guidance for parents; nine calendar months is shorter than 40 weeks in most month combinations.
- Dates are normalised before subtraction. That prevents daylight-saving changes, locale parsing quirks and timezone offsets from altering the clinical result.
- Negative corrected age is shown, not hidden. Before the due date, the honest output is a negative corrected age plus a positive PMA.
Where the milestone content comes from
The milestone prompts on this site are based on the 2022 CDC/AAP revision to developmental surveillance checklists — Zubler et al., Pediatrics 2022 (PMID 35132439), the paper behind CDC’s Learn the Signs. Act Early. milestone checklists. They are used here as conversation prompts, not as a screen, diagnosis or score. That distinction matters, because parents often encounter milestone content online that looks definitive when it is not.
To see how the milestone rows are applied after the age calculation, open the premature baby milestones chart. To see when the site treats concern as more important than the calendar, open the preemie red flags page.
Sources
Each row below links to the primary record — PubMed or DOI where one exists, otherwise the official publisher page — so a reader can open the original rather than trust a paraphrase.
| Source | Year | Role here | Limitation |
|---|---|---|---|
| AAP / HealthyChildren: Corrected Age for Preemies (aap.org) | current | Convention for correcting to about 2 years | A convention, not a rule; practice varies 18–36 months by domain |
| Zubler JM, Whitaker TM, et al. Pediatrics 2022;149(3):e2021052138 (summary: Am Fam Physician 2022;106(4):370-371) | 2022 | Milestone content used on this site | Surveillance prompts, explicitly NOT a validated screening instrument |
| Fenton TR, Kim JH. BMC Pediatrics 2013;13:59 (PMID 23601190) | 2013 | 22–50 week preterm growth reference | A descriptive reference of how preterm infants grew, not a prescriptive standard |
| Villar J, et al. INTERGROWTH-21st. Lancet Glob Health 2015;3(11):e681-e691 (PMID 26475015) | 2015 | Postnatal growth standard for preterm infants | Disagrees with Fenton at the margins, particularly below 33 weeks PMA |
| AAP Preterm Infant Growth Tools | current | Endorses both charts | Links out to charts; hosts no calculator |
| WHO Child Growth Standards | 2006 | Growth after term-equivalent age | Derived from term infants; a documented hand-off rule is required |
Fenton versus INTERGROWTH-21st
These two references disagree, and many tools quietly choose one without telling the user. AdjustedAge does not treat them as interchangeable. Fenton 2013 is a meta-analytic reference describing how preterm infants have actually grown, including those who grew poorly. INTERGROWTH-21st is a prescriptive standard describing how healthy preterm infants under optimal conditions grow.
A baby can sit on the 15th Fenton centile and below the 3rd INTERGROWTH centile at the same moment, and neither number is automatically wrong. That is why growth-chart plotting is treated more cautiously than a simple corrected-age calculator.
The current build deliberately waits to ship chart plotting until the presentation, source explanation and redistribution questions can all be handled properly. In other words, this page records not only what the site does, but what it refuses to simplify inaccurately.
Editorial review and correction policy
This site is reviewed by Dr. Zeeshan Islam, MBBS, MCPS (Pediatrics). Medical review here means more than adding a name to the footer. It means formulas, milestone framing, growth terminology and safety language are checked for clinical plausibility before publication.
If a formula, citation or clinical statement is found to be wrong, the rules governing it are corrected here, and the change itself is published as a dated entry in the corrections log on the about page — what was wrong, when it was fixed and why. Suspected errors can be reported through the contact details listed there.
What this tool deliberately will not do
- It will not return a pass, fail, score or percentile-of-development.
- It will not tell you your baby is fine. No tool can know that.
- It will not suppress the “act early” concerns at any age, in any state.
- It will not take formula-industry or infant-nutrition sponsorship.
- It will not send your data anywhere. Entries live in your browser's local storage and can be cleared by clearing site data. The privacy policy states exactly what is stored and what is not.
How this page differs from the worked formula guide
The worked formula page exists for a parent or clinician who wants to understand the arithmetic with examples. This methodology page exists for someone asking a different question: what assumptions does this website make, and are they stated clearly enough to trust?
The answer covers implementation details, source provenance, growth-chart standards, editorial limits and review policy — topics that do not belong in a basic how-to page.
Use the methodology with the calculator
Start with the corrected age calculator, then use the premature baby milestone chart for surveillance prompts. The worked examplesexplain the arithmetic, when to stop correctingexplains the domain-specific endpoints, and red flagsexplains when the calendar should stop being the main question.