Corrected age calculator for babies born prematurely
For healthcare professionals and students. Check results against your local protocols and clinical judgement; this tool does not give medical advice.
A baby born early is, in developmental terms, younger than the calendar says. This corrected age calculator works out a premature baby’s corrected (adjusted) age — the age counted from the due date — together with the chronological age since birth, the postmenstrual age and the due date itself. Enter the date of birth and the gestation at birth in weeks and days; the assessment date defaults to today.
Neonatal and community nurses, health visitors, paediatric trainees and therapists use corrected age to plot growth and judge milestones fairly, and parents often want to understand the number on a letter from clinic. The definitions follow the American Academy of Pediatrics, and the correction period follows UK-WHO growth chart guidance. Nothing is stored or sent.
How to use the Corrected Age Calculator (Premature Babies)
- Enter the baby’s date of birth.
- Enter the gestation at birth as completed weeks plus days — 31+3 is 31 weeks and 3 days.
- Leave Age on as today or choose the date of a clinic visit.
- Read the corrected age (or the postmenstrual age before the due date) and the chronological age, weeks premature and due date below it.
- Use Copy result to paste the ages and dates into notes.
Corrected Age Calculator (Premature Babies) features
All the perinatal ages
Corrected age, chronological age, postmenstrual age and due date from one set of inputs.
Weeks + days input
Gestation is entered the way it is written in notes (31+3), not as a decimal.
Before and after term
Shows postmenstrual age until the due date and corrected age from then on, as the AAP recommends.
Months and weeks
Corrected age in weeks and days for young babies, and in months and days (from the due date) for older ones.
Correction period note
Reminds you when a child is past the usual correction period on UK-WHO charts.
How corrected age is calculated
The American Academy of Pediatrics policy statement “Age Terminology During the Perinatal Period” (Pediatrics 2004;114(5):1362–1364) defines the terms used here. Gestational age is the time from the first day of the last menstrual period to birth. Chronological age is the time since birth. Postmenstrual age = gestational age + chronological age. Corrected age = chronological age − (40 weeks − gestational age at birth), and the statement recommends postmenstrual age rather than corrected age during the perinatal period, before term.
Two examples, as quoted from the AAP statement by the European PedCRIN neonatal age tool, are built into the self-test: a baby born at 27 weeks 3 days who is 12 weeks 6 days old has a postmenstrual age of 40 weeks 2 days; a baby born at 25 weeks who is 50 weeks old was born 15 weeks early and has a corrected age of 35 weeks.
Quick reference
| Born at | Weeks early | Chronological age 6 months | Corrected age |
|---|---|---|---|
| 36+0 weeks | 4 weeks | 26 weeks | about 22 weeks (5 months) |
| 32+0 weeks | 8 weeks | 26 weeks | about 18 weeks (4 months) |
| 28+0 weeks | 12 weeks | 26 weeks | about 14 weeks (3 months) |
| 24+0 weeks | 16 weeks | 26 weeks | about 10 weeks (2 months) |
How long to keep correcting
The UK guidance that accompanies the UK-WHO growth charts (Department of Health and RCPCH, 2009; checked 19 October 2026) says gestational correction is only for babies born before 37 weeks and “should continue until at least 1 year of age and until 2 years for infants born before 32 weeks”; measurements are plotted at actual age with a line drawn back by the number of weeks early. The AAP statement describes corrected age as used for children up to 3 years. Local services may differ, so follow your own charts and policy.
For parents: corrected age is about fairness, not a diagnosis. A baby born ten weeks early who is four months old is usually compared with babies of about seven weeks for smiling, head control and growth. Questions about a particular child’s development belong with the health visitor, GP or paediatric team.
Corrected Age Calculator (Premature Babies) FAQ
How do I calculate corrected age?
Subtract the number of weeks your baby was born early from their age since birth. Weeks early = 40 minus the gestation at birth. A baby born at 30 weeks is 10 weeks early, so at 20 weeks old their corrected age is 10 weeks. The corrected age calculator does this with exact days.
What is the difference between corrected age and adjusted age?
They mean the same thing: the age counted from the due date instead of the birth date.
What is postmenstrual age?
Gestational age at birth plus the time since birth. It is used in neonatal units before the due date; for example a baby born at 31+3 who is 4 weeks 6 days old is 36+2 weeks postmenstrual age.
Until what age should I use corrected age?
UK-WHO chart guidance continues correction until at least 1 year, and until 2 years for babies born before 32 weeks. The AAP describes corrected age as used up to 3 years. Follow your local guidance.
Does corrected age change vaccination dates?
No. Routine vaccines are normally given by chronological age, even for premature babies. Check your national schedule; there is a general vaccination schedule page for reference.
Is this corrected age calculator the same as the NHS one?
It uses the same definitions taught in the UK and elsewhere — chronological age minus weeks premature — but it is an independent tool. Always use the result your clinical team gives you if they differ.