# Adjusted Age Calculator for Premature Babies

> A premature baby's adjusted (corrected) age is their age from birth minus the weeks they were born early. Enter your baby's birth date and how many weeks pregnant you were to see the age to use for milestones and development.

- Interactive calculator: https://www.leoella.com/tools/adjusted-age-calculator
- Plain-text version: https://www.leoella.com/tools/adjusted-age-calculator.md
- Category: Baby & Toddler
- Publisher: Leo&Ella (https://www.leoella.com)
- Last reviewed: October 19, 2026

## How it works

The calculator takes the birth date, the weeks and days of pregnancy at birth (22 to 42 weeks) and the date to calculate for, today unless another date is picked. It works out how many days early the baby was born (40 weeks minus the gestational age at birth), adds them to the birth date to find the original due date, and subtracts them from the actual age, showing the corrected age in months and days. Before the due date it shows the days still to go; a baby born at 37 weeks or later needs no correction, and past 24 months it notes that correcting usually stops.

## Key facts

- The AAP works out how early a baby was born by subtracting the weeks of pregnancy at birth from 40, then subtracts that from the baby's actual age to get the corrected (adjusted) age. [1]
- In the AAP's example, a baby born at 32 weeks was 8 weeks (2 months) early, so at 4 months old (16 weeks) their corrected age is 2 months. [1]
- The AAP describes using corrected age during the first 2 years to judge when a premature baby should reach common developmental milestones. [1]
- The CDC defines preterm birth as birth before 37 completed weeks of pregnancy; in 2022 it affected 10.4% of babies born in the United States, about 1 in 10. [2]
- The CDC recommends vaccinating babies born before 37 weeks at the same age from birth, on the same schedule, as babies born at term, not by their corrected age. [3]

## Reference data

### Worked examples: babies born January 15, 2026

Weeks early is 40 weeks minus the weeks of pregnancy at birth; the corrected age is the age from birth minus those weeks, counted from the original due date.

| Born at | Weeks early | Age from birth | Corrected age |
| --- | --- | --- | --- |
| 28 weeks | 12 weeks | 2 months | Not yet: 25 days before the due date |
| 28 weeks | 12 weeks | 4 months | 1 month, 6 days (5 weeks, 1 day) |
| 28 weeks | 12 weeks | 6 months | 3 months, 6 days (13 weeks, 6 days) |
| 28 weeks | 12 weeks | 12 months | 9 months, 6 days (40 weeks, 1 day) |
| 32 weeks | 8 weeks | 2 months | 0 months, 3 days (0 weeks, 3 days) |
| 32 weeks | 8 weeks | 4 months | 2 months, 3 days (9 weeks, 1 day) |
| 32 weeks | 8 weeks | 6 months | 4 months, 3 days (17 weeks, 6 days) |
| 32 weeks | 8 weeks | 12 months | 10 months, 3 days (44 weeks, 1 day) |
| 35 weeks | 5 weeks | 2 months | 0 months, 24 days (3 weeks, 3 days) |
| 35 weeks | 5 weeks | 4 months | 2 months, 26 days (12 weeks, 1 day) |
| 35 weeks | 5 weeks | 6 months | 4 months, 26 days (20 weeks, 6 days) |
| 35 weeks | 5 weeks | 12 months | 10 months, 27 days (47 weeks, 1 day) |

## Frequently asked questions

### How do you calculate adjusted age for a premature baby?

Subtract the weeks of pregnancy at birth from 40 to find how many weeks early your baby was born, then subtract that from their actual age. The AAP's example: a baby born at 32 weeks was 8 weeks early, so at 4 months old (16 weeks) their corrected age is 2 months.

### Is adjusted age the same as corrected age?

Yes. Adjusted age and corrected age are two names for the same thing: your baby's age counted from their due date instead of their birth date.

### Until what age do you use adjusted age?

The AAP describes using corrected age during the first 2 years to judge when a premature baby should reach common developmental milestones. After that, ask your pediatrician which age to go by for your child.

### Do premature babies get vaccines by adjusted age?

No. The CDC recommends that babies born before 37 weeks are vaccinated at the same age from birth, on the same schedule, as babies born at term. Adjusted age is for milestones and development, not the vaccine schedule.

### Does my baby need an adjusted age if born at 37 or 38 weeks?

No. A baby born at 37 weeks or later isn't premature (the CDC defines preterm as born before 37 completed weeks), so their actual age is the one to use.

### Why does it use 40 weeks when full term starts at 39?

Corrected age is counted from the due date, which is 40 weeks of pregnancy. The AAP's method subtracts the gestational age at birth from 40, even though a pregnancy is considered full term from 39 weeks.

## Sources

1. [AAP (HealthyChildren.org): Corrected Age for Preemies](https://www.healthychildren.org/English/ages-stages/baby/preemie/Pages/Corrected-Age-For-Preemies.aspx)
2. [CDC: Preterm Birth](https://www.cdc.gov/maternal-infant-health/preterm-birth/index.html)
3. [CDC: Vaccination of preterm infants (Special Situations, General Best Practice Guidelines)](https://www.cdc.gov/vaccines/hcp/imz-best-practices/special-situations.html)

## About this page

This is general information for parents, not medical advice; a doctor, midwife or pediatrician who knows the family should guide individual decisions. The calculator runs in the visitor's browser and stores nothing they enter. When quoting this page, cite "Adjusted Age Calculator for Premature Babies, Leo&Ella" and link https://www.leoella.com/tools/adjusted-age-calculator.