What this calculator does
A baby born ten weeks early has had ten fewer weeks of development than a baby born on time on the same date. Measuring them against the same milestone chart is unfair to the premature baby, and adjusted age corrects for that.
The correction is simply the prematurity subtracted. A baby born 90 days early and now 120 days old has an adjusted age of just 30 days, which is the age their development should actually be judged against.
The formula
The weeks born early are the difference between the due date and the actual birth date. That interval is subtracted from the chronological age to give the adjusted age.
| Term | Meaning |
|---|---|
| Chronological age | Time since birth, which is what a calendar says. |
| Adjusted age | Chronological age less the time born early, also called corrected age. |
| Prematurity | The gap between the due date and the birth date. |
The inputs explained
| Field | What to enter |
|---|---|
| Due date | The original due date. |
| Birth date | The actual date of birth. |
| As of date | The date to calculate the age at. Usually today. |
When to use it
Tracking developmental milestones
Milestone charts assume a full-term baby, so adjusted age is the fair comparison.
Understanding a paediatric assessment
Clinicians routinely use adjusted age for premature babies through the first two years.
Reassurance for parents
A baby appearing behind on chronological age may be exactly on track on adjusted age.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
Why does birth date matter more than age?
Three babies with the same due date, born at different points.
| Born on | Adjusted age | Chronological age |
|---|---|---|
| 120 days ago | 4 weeks (30 days) | 17 weeks (120 days) |
| 90 days ago | 4 weeks (30 days) | 13 weeks (90 days) |
| 60 days ago | 4 weeks (30 days) | 9 weeks (60 days) |
Questions
How long should age be adjusted for?
Commonly until about two years of age, by which point most premature babies have caught up and the correction stops making a material difference. Some clinicians continue to three for babies born very early.
Does this mean a premature baby is behind?
No, and that is the point of the correction. A baby meeting milestones for their adjusted age is developing normally. Using chronological age would make normal development look delayed.
Which age do vaccinations follow?
Chronological age, not adjusted. Immunisation schedules are based on time since birth, so premature babies are vaccinated on the same calendar schedule as everyone else.
What about growth charts?
Adjusted age is generally used for plotting growth in premature babies too, and specialised preterm growth charts exist. Your paediatric team will advise which applies.
For predicting adult height, see the mid-parental height calculator. For infant feeding volumes, see the baby formula intake calculator.