The simple definitions
Chronological age is the easy one: it is the time since birth.Adjusted age — also called corrected age — subtracts the weeks of prematurity so the baby is compared against their due-date maturity instead. The AAP describes both terms as the same concept in preterm follow-up.
PMA, or postmenstrual age, is different again. It adds the gestational age at birth to the time since birth and is often useful in the earlier NICU and post-discharge period.
Which age should I use?
| Situation | Age to use | Why |
|---|---|---|
| Milestones and early development | Adjusted or corrected age | This is the fairer age for comparing a preterm baby to developmental expectations. |
| Routine vaccines | Chronological age | Routine immunisations usually follow time since birth, not the due-date age. |
| Everyday birthday counting | Chronological age | The calendar age is still the real age since birth. |
| Early NICU and neonatal notes | PMA may also be used | Postmenstrual age is often the more clinically useful number before the due date. |
| Growth and preterm follow-up discussions | Often adjusted or corrected age | The early interpretation is usually linked to due-date maturity rather than birthday alone. |
Vaccine timing follows the CDC immunisation schedules and the AAP Red Book chapter on immunisation in preterm and low birth weight infants; milestone timing follows the AAP's preemie milestones and the CDC's Learn the Signs. Act. Early. milestones. Full detail sits on the preemie vaccines guide.
Why the difference matters
If you use chronological age for a milestone conversation, a preterm baby may look delayed simply because the wrong calendar was used. If you use corrected age for routine vaccines, the baby may be left unprotected for longer than necessary — which is why the AAP gives preterm immunisation by chronological age and birth weight rather than by due date. The age framework changes the conclusion, which is why mixing them up causes so much confusion.
What adjusted age actually changes
Adjusted age mainly changes the interpretation of development and some follow-up growth discussions. It does not erase the real time since birth. Both ages are true — they just answer different questions.
Use the adjusted age calculator if that is the wording you searched for, or the PMA calculator if you are reading an early NICU note.
Common questions
Is adjusted age the same as corrected age?
Yes. They are two names for the same concept in preterm follow-up.
What is chronological age?
Chronological age is simply the baby's real age from the day of birth.
Which age should I use for milestones?
For early follow-up, milestones are usually interpreted using adjusted or corrected age. Use the premature baby milestones chart after checking the calculator.
Which age should I use for vaccines?
Routine vaccines usually follow chronological age. See preemie vaccines.
Sources for this page
- HealthyChildren.org (AAP) — Corrected Age For Preemies
- HealthyChildren.org (AAP) — Preemie Milestones
- HealthyChildren.org (AAP) — Immunizations for Preterm Babies
- AAP Red Book — Immunization in Preterm and Low Birth Weight Infants
- CDC — Recommended Immunization Schedules
- CDC — Learn the Signs. Act. Early. Milestones
Methodology and full source list: how we build these pages. Corrections are logged publicly on our corrections log.