Why corrected age changes the percentile
A baby born 8 weeks early who is 4 months old has had the same time to grow as a 2-month-old born at term. On a growth chart, that baby belongs next to other 2-month-olds. Plotted at 4 months, the same weight can look like a low percentile; plotted at 2 months corrected, it's often right in the normal range. The AAP recommends corrected age for the first 2 years.
The calculator subtracts the weeks of prematurity (40 minus the gestational age at birth) from your baby's actual age, then looks up the WHO percentile at both ages. Babies born at 37 weeks or later are full term and don't need correcting. For the bigger picture, read our guide to premature baby growth and how catch-up growth works.
Corrected age examples
| Born at | Weeks early | Actual age | Corrected age |
|---|---|---|---|
| 36 weeks | 4 weeks | 3 months | about 2 months |
| 32 weeks | 8 weeks | 4 months | about 2 months |
| 28 weeks | 12 weeks | 6 months | about 3¼ months |
| 28 weeks | 12 weeks | 12 months | about 9¼ months |
Method: AAP, HealthyChildren.org. Months are approximate (1 month ≈ 4.35 weeks); the calculator works in days.
Frequently asked questions
Should I use corrected age for my preemie's growth chart?▾
Yes. For a baby born early, growth and development are judged by corrected age for the first 2 years. Plotting a preemie at actual age compares them with babies who had weeks more time to grow, which makes their percentile look lower than it really is.
How is corrected age calculated?▾
Subtract the weeks of prematurity from your baby's actual age. Weeks of prematurity are 40 minus the gestational age at birth. A baby born at 32 weeks is 8 weeks early, so at 16 weeks old (4 months) their corrected age is 8 weeks (2 months).
Until what age do you correct for prematurity?▾
Corrected age is used for the first 2 years. By then most preemies have caught up, and the few weeks' difference matters less and less as a child gets older. Your pediatrician may use a different cutoff for babies born very early. Vaccines always follow actual (chronological) age.
Which growth chart does the NICU use?▾
Before the due date, NICU teams use preterm growth charts such as the Fenton chart (to 50 weeks postmenstrual age) or the INTERGROWTH-21st preterm standards (to 64 weeks). After that, babies move to the WHO charts plotted by corrected age. That's why this calculator only gives WHO percentiles once your baby has reached their due date.
When will my preemie catch up?▾
Head size usually catches up first, followed by weight and length, and most catch-up growth happens by age 2 to 3. Some children who were born very early or very small stay on a lower curve. A percentile that holds steady or rises at corrected age is the pattern to look for.
Sources
- AAP (HealthyChildren.org): Corrected age for preemies
- Fenton & Kim (2013): A revised preterm growth chart harmonized with the WHO standards, BMC Pediatrics
- Villar et al. (2015): INTERGROWTH-21st postnatal growth standards for preterm infants, Lancet Global Health
- AAFP (1998): Office care of the premature infant, part II (catch-up growth)
- CDC MMWR (2010): Use of WHO and CDC growth charts for children aged 0–59 months in the United States
- WHO Child Growth Standards