Corrected age calculator
Enter the birth date and how many weeks gestation your baby was born at.
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How do you calculate corrected age?
Subtract the number of weeks your baby was born early from their chronological age. A baby born at 32 weeks was 8 weeks early, so at 6 months old chronologically their corrected age is 4 months. Correction is normally applied until about 2 years of age, by which point most children have caught up.
Why milestones use corrected age
A baby born at 32 weeks has had eight fewer weeks of development than a baby born at term on the same day. Judging them against the same milestone chart sets them up to look behind when they are not.
Corrected age puts them back on a fair footing. It answers the question milestone charts are actually asking — how long has this baby been developing — rather than how long they have been outside.
This is why neonatal follow-up clinics work in corrected age as a matter of routine, and why a parent comparing their preemie against a chronological chart usually ends up unnecessarily worried.
The calculation
Full term is counted as 40 weeks.
weeks premature = 40 − gestational age at birthcorrected age = chronological age − weeks prematureborn at 32 weeks → 8 weeks prematurechronological 6 months → corrected 4 monthscorrection is normally applied until about 2 years corrected
The original due date is simply the birth date plus the weeks of prematurity, and it is often the more intuitive way to think about it: corrected age is the baby's age counted from when they were due rather than when they arrived.
Correction is conventionally stopped at around two years, when most children born preterm have closed the gap. Some follow-up programmes continue further for babies born very early, which is a clinical judgement rather than a rule.
Worked example: born at 30 weeks
A baby born at 30 weeks and 2 days, now 7 months old chronologically.
- Full term40 weeks
- Born at30 weeks 2 days
- Weeks premature9.7 weeks
- Chronological age7 months
- Less 9.7 weeks≈ 4 months 21 days corrected
Developmentally this baby sits around 4 months 3 weeks, not 7 months.
That is a substantial difference at this stage. A 7-month milestone chart would flag this baby as delayed; a corrected-age chart shows them progressing normally.
What corrected age is used for
- Developmental milestones — sitting, rolling, babbling, walking
- Growth charts, where corrected age is plotted rather than chronological
- Starting solids, which follows developmental readiness rather than the calendar
- Neonatal follow-up appointments and assessments
- Setting expectations, which is often the part that matters most to parents
Immunisations are the notable exception: they are given on chronological age, not corrected, because the immune system responds on its own timetable.
Assumptions and limits
- Uses 40 weeks as full term, the standard convention
- Assumes an accurately dated gestational age at birth
- Correction is stopped at about two years, which is a convention rather than a hard rule
- Says nothing about any individual baby's actual development
- Immunisation schedules follow chronological age and are not adjusted
Common questions
What is corrected age?
A preterm baby's age adjusted for how early they arrived. Subtract the weeks of prematurity from the chronological age. A baby born at 32 weeks is 8 weeks early, so at 6 months chronological their corrected age is 4 months.
How long do you use corrected age?
Usually until about two years, by which point most children born preterm have caught up with their term-born peers. Some follow-up programmes continue longer for babies born very early, which is a clinical decision.
Should immunisations use corrected age?
No. Vaccines are given according to chronological age, not corrected age, because the immune system develops on its own schedule. This is the main exception to the general rule.
My baby was born at 37 weeks — do I need to correct?
Correction is usually applied for babies born before 37 weeks. At 37 weeks and beyond a baby is considered early term, and the three-week adjustment rarely makes a meaningful difference to milestone assessment.
Is my baby behind if they are meeting corrected milestones?
No. Meeting milestones at corrected age is exactly what is expected of a baby born preterm. That is the whole reason corrected age exists. Any specific concern is worth raising with your paediatrician rather than a chart.
Where these numbers come from
- Canadian Paediatric Society — Caring for babies born preterm Canadian guidance on corrected age and preterm follow-up.
- Government of Canada — Child growth and development National resources on developmental milestones.
Last verified 2026-08-01 The method on this page is checked against the sources above at least once a year. Spotted something out of date? Tell us and we will fix it.
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