Adjusted Age Calculator for Premature Babies (corrected age)

Understanding the adjusted age calculator — concept and principle

This calculator works out the corrected (adjusted) age of a baby born prematurely, together with the chronological age, the number of weeks born early, the postmenstrual age, the original due date and the date when the baby reaches a corrected age of two years. Parents of preterm babies and health visitors use it to judge growth and milestones fairly.

Subtract the weeks born early — with the default inputs: 9.1 weeks.

How it is calculated

Weeks born early = 40 weeks − gestational age at birth
Chronological age = days since the birth date
Corrected age = chronological age − time born early
Postmenstrual age = gestational age at birth + chronological age
Original due date = birth date + time born early
Months are counted as 30.44 days

The principle

A baby born eight weeks early has had eight fewer weeks of development in the womb. Comparing that baby with term babies of the same chronological age would make normal progress look slow. Corrected age removes the head start term babies had, so growth charts and developmental milestones are read against babies at the same stage. The American Academy of Pediatrics defines these age terms; paediatricians usually use corrected age for babies born before 37 weeks until about two years of age. A negative corrected age means the original due date has not yet been reached.

Worked example

Defaults: born at 32 weeks + 0 days, 120 days before the calculation date (shown here for 10 November 2026).

Born early by = 40 − 32 = 8 weeks (56 days).
Chronological age = 120 days = 17.1 weeks = 3.9 months.
Corrected age = 120 − 56 = 64 days = 9.1 weeks = 2.1 months.
Postmenstrual age = (224 + 120) ÷ 7 = 49.1 weeks.
Original due date = Mon, Sep 7, 2026; corrected age 2 years on Thu, Sep 7, 2028.

The dates move with the birth date you enter; the week figures stay the same for the same gap.

How to read the result

Use the corrected age to plot weight, length and head size and to compare with milestones such as smiling, sitting or walking. Many preterm babies catch up during the first two years. Bring questions about growth or development to your baby's health team, who will consider the whole picture.

Common mistakes

Using chronological age on growth charts for a preterm baby.
Forgetting the extra days of gestation (32 + 4 is not 32).
Continuing to correct well beyond the age your health team uses.
Using corrected age for vaccinations, which usually follow chronological age.

Keep in mind: This is not a diagnosis or a developmental assessment. Vaccination schedules and medical decisions follow your health team's advice.

Frequently asked questions

How do I calculate my preemie's adjusted age?

Subtract the number of weeks born early (40 minus gestational age) from the age since birth.

Until what age should I use corrected age?

Usually until about 2 years, but follow your paediatrician's advice.

Do vaccines follow corrected age?

Vaccines are usually given by chronological age; check with your health team.

Sources: AAP Committee on Fetus and Newborn, "Age Terminology During the Perinatal Period", Pediatrics 2004;114(5):1362–1364 · AAP HealthyChildren.org, "Corrected Age For Preemies"

Checked 9 Nov 2026. AAP 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); AAP HealthyChildren.org, “Corrected Age For Preemies”.

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