Corrected age calculator for preemies
A premature baby’s growth and milestones are judged by corrected age, the age they would be if born on the due date, until about 2 years.
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How to use it
- Enter date of birth, gestation at birth: weeks, gestation at birth: days and age on.
- The result appears straight away and updates as you edit.
- Copy the result, or copy a link that keeps the same values to share it.
How it is calculated
Days early = 280 - (weeks at birth × 7 + days). Original due date = birth date + days early. Corrected age = chosen date - due date. Before the due date the tool shows postmenstrual age = age at birth + days since birth.
Example
Inputs: Date of birth = 2026-03-01, Gestation at birth: weeks = 32 weeks, Gestation at birth: days = 0 days, Age on = 2026-09-30. Result: Corrected age: 5 months 4 days; Actual age: 6 months 29 days; Born early by: 8 weeks.
Limits and caveats
- Corrected age is usually used until age 2, after which actual age applies.
- Most schedules give vaccines by actual age, not corrected age.
Questions about this tool
Why not judge growth by actual age?
The baby was born before finishing growth in the womb, so comparing without correction to term babies is unfair.
How long should I use corrected age?
The American Academy of Pediatrics suggests using it until about age 2 for most premature babies.
What is postmenstrual age?
It is the gestational age at birth plus the days since birth, used before the original due date is reached.