A premature baby’s corrected (adjusted) age from the birth date and gestational age, using the AAP method, plus the due date and when correcting usually stops
Updated October 2026
A baby born early has two important dates: the birthday and the due date. The American Academy of Pediatrics uses this formula:
Put another way, corrected age is the time since the due date.
AAP’s worked example. A baby born at 32 weeks was 8 weeks (2 months) early. At 4 months old (16 weeks since birth), her corrected age is 16 − 8 = 8 weeks, or 2 months. She should be expected to have the skills of a 2-month-old born at term: just starting to hold her head up and smile, not yet rolling over. The calculator’s default reproduces this with a June 1, 2026 birth at 32 weeks, checked on September 21, 2026. AAP counts months as 4-week months, so the calculator shows both weeks and 4-week months.
| Rule | Detail | Source |
|---|---|---|
| Who gets a corrected age | Babies born more than 3 weeks early (before 37 weeks) | AAP HealthyChildren (2024) |
| Reference point | 40 weeks; “full term” is now considered 39 weeks | AAP HealthyChildren (2018) |
| Milestones and growth | Use corrected age until 2 years; by then most children have caught up | AAP HealthyChildren (2024) |
| Developmental test scores | Corrected up to 3 years in AAP guidance, though practice varies | Wilson-Ching et al., 2014 |
| CDC milestone checklists | The CDC Milestone Tracker app uses corrected age for babies born prematurely | CDC (updated Feb. 16, 2026) |
| Before the due date | Babies still in the NICU are described by postmenstrual age (gestational age + weeks since birth) | AAP Committee on Fetus and Newborn, 2004 |
The AAP’s 2004 policy statement on age terminology defines gestational age, postmenstrual age, chronological age and corrected age so that growth and developmental results can be compared consistently. Correcting matters most early on. A 2014 modelling study found that using actual age instead of corrected age lowers a preterm child’s scores most at younger ages and for babies born earliest.
| Baby | Actual age | Born early by | Corrected age |
|---|---|---|---|
| Born at 32 weeks, now 16 weeks old (AAP example) | 16 weeks 0 days | 8 weeks 0 days | 8 weeks 0 days (2.0 mo) |
| Born at 28 weeks, now 6 months (26 weeks) old | 26 weeks 0 days | 12 weeks 0 days | 14 weeks 0 days (3.5 mo) |
| Born at 34 weeks 3 days, now 1 year old | 52 weeks 1 day | 5 weeks 4 days | 46 weeks 4 days (11.6 mo) |
| Born at 25 weeks, now 18 months old | 78 weeks 2 days | 15 weeks 0 days | 63 weeks 2 days (15.8 mo) |
| Born at 36 weeks, now 4 months (16 weeks) old | 16 weeks 0 days | 4 weeks 0 days | 12 weeks 0 days (3.0 mo) |
| Born at 30 weeks, now 6 weeks old | 6 weeks 0 days | 10 weeks 0 days | Before due date (PMA 36 weeks 0 days) |
The gap between actual and corrected age never shrinks: a baby born 15 weeks early is always 15 weeks “younger” by the corrected count. What shrinks is how much those weeks matter. Fifteen weeks is more than half the life of a 6-month-old, but a small fraction of a 3-year-old’s.
Plot a preterm baby’s weight, length and head size at the corrected age, and compare milestones with those of a term baby of that corrected age. A baby of 4 months actual age and 2 months corrected age who is just beginning to smile and hold up her head is on track. Family members who compare her with other 4-month-olds may worry when there is no reason to. Watch for steady progress from one skill to the next, such as pulling up, then standing, then walking.
This calculator does the date arithmetic only; it does not assess development. Bring your questions to your pediatrician or NICU follow-up clinic in these cases:
Ask too about early intervention services, which AAP notes some babies receive to help them develop. Feeding, sleep, vaccines and medication doses have their own rules, so follow your care team’s instructions for those.