Paediatric Corrected Age Calculator
Paediatric Corrected Age Calculator
Corrected age from chronological age and gestational age at birth — the age a preterm baby must be assessed against for growth, development and many reference intervals, because chronological age makes a normal preterm baby look abnormal.
Corrected age for prematurity
Chronological age + gestation → corrected ageA baby born at 32 weeks of gestation, now 16 weeks old
Formula
corrected age = chronological age − weeks of prematurity
postmenstrual age = corrected age + 40 weeks = gestational age at birth + chronological age
- 40 weeks
- the convention. The AAP’s guidance subtracts the number of weeks the baby was preterm from the chronological age, where prematurity is measured from 40 weeks, and published growth work uses 40 weeks or 280 days for the same purpose. Some units correct from 37 weeks or from the estimated due date instead, which shifts the answer by up to three weeks — worth knowing which convention a chart or a report is using
- corrected age
- also called adjusted age or, loosely, corrected gestational age. It is the age the child would be had they been born at term, and it is what growth charts and developmental milestones after term-equivalent should be read against
- postmenstrual age
- gestational age at birth plus chronological age — the more useful figure before term-equivalent, and what a neonatal unit means when it describes a baby as ‘now 34 weeks’. It is simply corrected age plus 40
- why weeks and not months
- because the convention is written in weeks and month arithmetic introduces error. A month is 4.345 weeks, not 4, so a ‘4-month-old’ is 17.4 weeks rather than 16 — a 1.4-week discrepancy that is a sixth of the correction for a 32-week baby
- two years
- the conventional duration of correction for growth and developmental assessment, and what the AAP’s patient-facing guidance describes
- three years
- the better-supported figure for the most preterm. Published analysis of preterm growth reports that correcting to two years is the commonest practice but concludes correction is required for all growth measures through 36 months of corrected age in extremely and very preterm children
- the exception
- immunisation, which follows chronological age with no correction for prematurity. Correcting it would delay protection in the babies who need it soonest
Worked example
A baby born at 32 weeks of gestation, now 16 weeks old
Weeks of prematurity = 40 − 32 = 8 weeks
Corrected age = 16 − 8 = 8 weeks, or about 2 months
This is the AAP's own worked example: a baby born 8 weeks early, at 4 months of age, has a corrected age of 2 months and should be assessed against what a 2-month-old does
Postmenstrual age = 32 + 16 = 48 weeks, which is the same thing expressed the other way: 8 weeks corrected plus 40
The correction is absolute and never changes. At 52 weeks chronological the same child is 44 weeks corrected; at two years chronological, 96 weeks corrected. What changes is how much of the child's life 8 weeks represents — half of it at 16 weeks, a twelfth of it at two years
A baby born at 26 weeks has a 14-week correction, so at 16 weeks chronological their corrected age is 2 weeks — they have only just reached term-equivalent, and until they did, postmenstrual age was the figure to use
Immunisations for all three of these babies are due at 8 weeks of CHRONOLOGICAL age, not corrected. That is the one place the correction must not be applied
Correction by gestational age at birth
| Born at | Weeks preterm | Corrected age at 16 weeks chronological | Corrected age at 1 year chronological |
|---|---|---|---|
| 24 weeks | 16 | 0 weeks — just term-equivalent | 36 weeks (about 8 months) |
| 26 weeks | 14 | 2 weeks | 38 weeks |
| 28 weeks | 12 | 4 weeks (about 1 month) | 40 weeks (about 9 months) |
| 30 weeks | 10 | 6 weeks | 42 weeks |
| 32 weeks | 8 | 8 weeks (about 2 months) | 44 weeks (about 10 months) |
| 34 weeks | 6 | 10 weeks | 46 weeks |
| 36 weeks | 4 | 12 weeks (about 3 months) | 48 weeks (about 11 months) |
| 40 weeks | 0 | 16 weeks | 52 weeks |
Where corrected age is used, and where it is not
| Assessment | Which age |
|---|---|
| Weight, length and head circumference on a growth chart, after term-equivalent | Corrected age, conventionally to 2 years and arguably to 3 in the very preterm |
| Growth before term-equivalent | Postmenstrual age, on a preterm or intrauterine growth reference |
| Developmental milestones — sitting, walking, speech, social | Corrected age, to about 2 years |
| Neonatal reference intervals and screening schedules on the unit | Postmenstrual age |
| Retinopathy of prematurity screening | Postmenstrual age, alongside chronological age criteria in the national protocol |
| Immunisations | CHRONOLOGICAL age, with no correction for prematurity |
| Drug dosing by weight | Neither — the current weight, though postmenstrual age drives clearance for many neonatal drugs |
Converting between weeks and months without introducing error
| Weeks | Months (weeks ÷ 4.345) |
|---|---|
| 4 weeks | 0.9 months |
| 8 weeks | 1.8 months |
| 13 weeks | 3.0 months |
| 26 weeks | 6.0 months |
| 52 weeks | 12.0 months |
| 104 weeks | 23.9 months |
| 156 weeks | 35.9 months |
Eight weeks that do not disappear
A baby born at thirty-two weeks of gestation is eight weeks short of term. Those eight weeks do not go away. At four months of age the baby has had sixteen weeks outside the womb but only eight weeks of development beyond the point a term newborn starts from, and every assessment that assumes otherwise will find them wanting. Corrected age is the arithmetic that fixes this: subtract the weeks of prematurity — forty minus the gestational age at birth — from the chronological age, and compare the child against that. It is the simplest calculation on this site, and one of the ones most often skipped, usually because whoever is doing the assessment was handed a date of birth and nothing else.
The consequence of skipping it is systematic and predictable: a normal preterm baby looks abnormal. Plotted at chronological age on a term growth chart, a healthy twenty-eight-week baby tracks along or below the bottom centile and appears to be failing to thrive. Assessed at chronological age against developmental milestones, the same baby appears delayed in every domain. Both readings are artefacts of the wrong denominator. They generate unnecessary referrals, unnecessary investigation and a great deal of avoidable parental anxiety — and, more insidiously, they can normalise concern to the point where a genuine problem in a preterm child is dismissed as expected. The correction works in both directions: it prevents false alarm, and it preserves the ability to recognise real abnormality.
Before term-equivalent, corrected age is negative and the useful figure is postmenstrual age — gestational age at birth plus chronological age, which is simply corrected age plus forty weeks. That is what a neonatal unit means when it says a baby is ‘now thirty-four weeks’, and it is the axis for preterm growth charts, for retinopathy of prematurity screening and for a good many neonatal reference intervals, because the biology being measured is maturational rather than postnatal. Haemoglobin, reticulocyte count, creatinine and alkaline phosphatase all behave differently in a baby of thirty-four weeks postmenstrual age than in a term newborn of the same postnatal age, and drug clearance for many neonatal medicines is modelled on postmenstrual age for the same reason.
Two questions of convention remain, and both deserve an honest answer rather than a confident one. The first is how long to correct. Two years of corrected age is the commonly used stopping point and is what the American Academy of Pediatrics describes to parents, but published analysis of preterm growth reports important misclassification of growth beyond that and concludes that correction is required for all growth measures through thirty-six months of corrected age in extremely and very preterm children. Two years is the convention; three is better supported for the most preterm. The second is the exception, and it is the one that matters most in practice: immunisation follows chronological age with no correction whatsoever. A baby born at twenty-eight weeks has their first immunisations at eight weeks of chronological age, on the neonatal unit if that is where they still are. Correcting an immunisation schedule for prematurity delays protection in precisely the infants least able to withstand the infections being vaccinated against, and the guidance says so plainly: preterm babies follow the same schedule with no correction for their prematurity.
Frequently asked questions
How do you calculate corrected age for a premature baby?
Subtract the weeks of prematurity from the chronological age, where weeks of prematurity is 40 minus the gestational age at birth. A baby born at 32 weeks is 8 weeks preterm, so at 16 weeks of age their corrected age is 8 weeks, or about 2 months. Postmenstrual age is the same thing plus 40 weeks: 32 + 16 = 48 weeks.
How long should corrected age be used?
Two years of corrected age is the conventional stopping point for growth and developmental assessment and is what the AAP’s guidance describes. Published analysis of preterm growth finds important misclassification up to three years and concludes that correction is required for all growth measures through 36 months of corrected age in extremely and very preterm children, so for the most preterm children correcting to three years is better supported.
What is the difference between corrected age and postmenstrual age?
They differ by exactly 40 weeks. Postmenstrual age is gestational age at birth plus chronological age, and is the figure used while a baby is still below term-equivalent — it is what a neonatal unit means by ‘now 34 weeks’. Corrected age is postmenstrual age minus 40 weeks, and becomes the useful figure once the baby has passed term-equivalent.
Should immunisations be given at corrected age?
No. Preterm babies are immunised according to chronological age with no correction for prematurity, so a baby born at 28 weeks has their first immunisations at 8 weeks of chronological age, on the neonatal unit if necessary. Correcting for prematurity here would delay protection in the babies most vulnerable to the infections being vaccinated against.
Why does using chronological age make a preterm baby look abnormal?
Because it compares them against children who have had more time to develop. A healthy 28-week baby plotted at chronological age on a term growth chart appears to be failing to thrive, and assessed at chronological age against milestones appears globally delayed. Both are artefacts of the wrong age, and both generate unnecessary referral and investigation.
Is corrected age calculated from 40 weeks or from the due date?
This calculator uses 40 weeks, which is the AAP convention and the one used in published growth analyses. Some units correct from 37 weeks or from the estimated date of delivery instead, which can shift the answer by up to three weeks — so it is worth knowing which convention a particular chart or report assumes.
Related calculators
References
- American Academy of Pediatrics. Corrected Age for Preemies. HealthyChildren.org — subtracting the weeks a baby was preterm from chronological age, and using corrected age through the first 2 years for developmental goals.
- Sauve R, Lee SK, et al. Preterm growth assessment: the latest findings on age correction. J Perinatol. 2025 — correction to 2 years as commonest practice, with age correction required for all growth measures through 36 months of corrected age in extremely and very preterm children.
- Engle WA; American Academy of Pediatrics Committee on Fetus and Newborn. Age terminology during the perinatal period. Pediatrics. 2004;114(5):1362–1364 — definitions of gestational, chronological, postmenstrual and corrected age.
- NHS Greater Glasgow and Clyde. Immunisation Guideline for Neonates. Paediatric clinical guideline 523 — preterm babies follow the same immunisation protocol with no correction for their prematurity.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.