Adjusted Age Calculator

This calculator works out a premature baby's adjusted age, also called corrected age, which counts their age from the original due date rather than the day they were actually born. When a baby arrives early they have had less time to grow than a full-term baby of the same calendar age, so tracking milestones like rolling, sitting and first words against corrected age gives a fairer and more reassuring picture of how they are developing. Enter the date of birth, the gestational age at birth in weeks, and the date you want the age worked out for, and the calculator returns the corrected age in both weeks and months, alongside the chronological age so you can see the difference at a glance. It works by taking full term as 40 weeks, finding how many weeks early the baby was, and subtracting that gap from the chronological age. Parents, midwives and Plunket nurses use corrected age when reading growth charts and developmental checklists during the first couple of years. Correction is usually applied until around two years of age, by which point most premature babies have caught up with their peers. This is a general tracking tool and not medical advice, so always follow the guidance of your midwife, Plunket nurse or paediatrician.

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weeks
19 weeks 0 days
corrected (adjusted) age
Chronological age27 weeks 0 days
Born early by8 weeks
Corrected age in months4.4 months

Correction is usually applied until about two years of age. Full term is taken as 40 weeks.

General tracking tool only, not medical advice. Follow your midwife, Plunket nurse or paediatrician for developmental checks.

How it works

The chronological age is simply the number of days between the date of birth and the date you are checking, which the calculator converts into weeks and days. The number of weeks premature is 40 weeks (full term) minus the gestational age at birth, converted to days by multiplying by seven. The corrected age is then the chronological age in days minus those prematurity days. If a baby was born at or after 40 weeks the correction is zero, so the corrected age equals the chronological age. Months are worked out by dividing the corrected age in days by 30.44, the average length of a month, which is why the months figure is a decimal rather than a whole number.

Worked example

A baby is born on 15 January 2026 at 32 weeks gestation, so they are 40 minus 32, which is 8 weeks premature. Checking their age on 23 July 2026, the chronological age is 189 days, or exactly 27 weeks. The 8 weeks premature is 56 days, so the corrected age is 189 minus 56, which is 133 days, or 19 weeks. Dividing 133 by 30.44 gives about 4.4 months. So while the calendar says this baby is nearly seven months old, developmentally you would expect them to behave more like a four and a half month old, which is exactly why milestones are read against the corrected age.

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Frequently asked questions

What is adjusted or corrected age?

Adjusted age, also called corrected age, is a baby's age counted from the original due date rather than the actual birth date. For a baby born early, you subtract the number of weeks they were premature from their chronological age. It gives a fairer picture of expected development, because a baby born eight weeks early has had eight fewer weeks to grow than a full-term baby of the same chronological age.

How do you calculate corrected age?

First find the weeks premature: 40 weeks (full term) minus the gestational age at birth. Then subtract that from the chronological age. A baby born at 32 weeks is 8 weeks premature, so at a chronological age of 27 weeks their corrected age is 27 minus 8, which is 19 weeks.

Until what age do you correct for prematurity?

Most guidance corrects age when tracking growth and developmental milestones until about two years old, by which time most premature babies have caught up. Some clinicians continue correcting for very premature babies a little longer. This is a general tracking tool, so follow the advice of your midwife, Plunket nurse or paediatrician.