How corrected age works
A baby born prematurely has a birth date and an expected due date. Chronological age counts time from birth; corrected age counts from the expected due date. Seeing both dates helps explain why a premature baby can have two different age descriptions.
Corrected age formula
Corrected age = chronological age − (40 weeks − gestational age at birth)
The calculation uses 40 weeks as its reference point. Enter gestational age as completed weeks plus additional days so that a birth at 32 weeks and 3 days is handled precisely. Postmenstrual age uses a different sum: gestational age at birth plus chronological age.
Example: born at 32 weeks
A baby born at 32 weeks is 8 weeks before the 40-week reference. At 16 weeks after birth, chronological age is 16 weeks, corrected age is 8 weeks, and postmenstrual age is 48 weeks. Weeks are exact seven-day units; calendar months have different lengths.
A timeline and calendar you can explore
The interactive result places birth, the expected due date and your selected assessment date on one timeline. Move the date selector to compare ages on another day, or select a calendar row to explore a corrected-age anniversary. The calendar uses actual calendar months, rather than treating every month as four weeks.
Growth and developmental context
AAP parent guidance describes using corrected age to understand development during the first two years. Its growth assessment guidance recommends age correction for children born preterm until age three. These are different uses; the calculator does not apply one universal stopping date or assess developmental delay.
A birth date and gestational age cannot predict an individual baby’s future weight, length or developmental achievements. Growth charts need actual measurements and a suitable reference. Discuss developmental concerns and the choice of growth chart with the baby’s pediatrician.
Frequently asked questions
What is the difference between corrected and chronological age?
Chronological age is the time since birth. Corrected age subtracts the interval between birth and the expected due date. Both ages describe the same baby from different starting dates.
Why is the result before the due date different?
Before the expected due date, the mathematical corrected age is negative. This calculator shows how much time remains until that date instead. Postmenstrual age is still shown in weeks and days.
Can I calculate corrected age using a due date?
Yes. Select the expected due date method and enter the original expected date of delivery. The calculator derives the gestational age implied by that date, using the 40-week reference.
Why are weeks and calendar months different?
A week always has seven days, but calendar months have 28 to 31 days. The calculator shows exact weeks and days alongside calendar months and days, without using a four-week approximation.
Does the calendar predict when my baby will walk or talk?
No. It only shows when the baby reaches selected corrected calendar ages. Development varies, so these dates are not deadlines for milestones or proof of normal or delayed development.
Can I share or print the result?
Yes. Open the result and choose Copy or Print. A link contains the dates used in the calculation and works without an account. Anyone who receives the link can see those dates.
Sources and related tools
- AAP · Read AAP guidance for parents
- AAP · Term Infant Growth Tools
- NICE NG72 · Developmental follow-up of children born preterm
- Open growth calculator
Formula and guidance checked on 8 October 2026.