Corrected age calculator
Corrected age is a preterm baby's age counted from the due date instead of the birth date: the age since birth minus the weeks born early. Enter the birth date and how many weeks early your baby arrived to see both ages, the CDC milestone checklist to use today, and the date correction stops (the second birthday).
Corrected age calculator
Weeks early = 40 minus the weeks of pregnancy at birth. Everything is calculated in this browser; nothing is sent or saved.
Example: a child born April 15, 2025, checked on October 15, 2026 (18 months old). Enter your child's birth date.
- Corrected age
- 16 months 5 days
- about 16 months, counted from the due date, June 10, 2025
- Chronological age
- 18 months
- about 18 months, counted from the birth date
- Stop correcting on
- April 15, 2027
- the second birthday (24 months chronological)
Born 8 weeks early means born at about 32 weeks of pregnancy, so the corrected age runs 8 weeks (about 2 months) behind the age since birth. Until April 15, 2027, compare skills with the corrected age: between two checklist ages CDC says to use the earlier one, which is the 15-month checklist here. See what is emerging at 16 months.
When each CDC checklist applies for this child
How to calculate corrected age
- Find the weeks born early. A due date is set at 40 weeks of pregnancy, so weeks early = 40 minus the weeks of pregnancy at birth. Born at 32 weeks means 8 weeks early.
- Work out the age since birth (chronological age), in months and days.
- Subtract the weeks early. 8 weeks is a little under 2 months. The quicker route is to count the age from the due date, which is what the calculator does.
- Pick the checklist. Use the latest CDC checklist age at or below the corrected age: 12, 15, 18, 24, 30 or 36 months on this site.
Worked examples
Three children checked on October 15, 2026, calculated with the same code as the tool above:
| Born early | Age since birth | Corrected age | Checklist to use |
|---|---|---|---|
| 4 weeks (at 36 weeks) | 13 months | 12 months 2 days | 12-month checklist |
| 10 weeks (at 30 weeks) | 18 months | 15 months 21 days | 15-month checklist |
| 14 weeks (at 26 weeks) | 25 months | not used after 24 months | 24-month checklist |
A 13-month-old born 4 weeks early is 12 months 2 days corrected, so the 12-month checklist is the fair comparison. A child born 10 weeks early who has just turned 18 months is 15 months 21 days corrected: the 15-month checklist applies until December 24, 2026, when the 18-month checklist takes over. The third child, born 14 weeks early, is 25 months old: past the second birthday, so the chronological age and the 24-month checklist apply even though the corrected age would be lower.
When to stop correcting
The American Academy of Pediatrics advises using corrected age when you judge development during the first two years, then switching to the age since birth. Many preterm babies catch up with their full-term peers by about age 2, which is why the correction ends there. The calculator shows the exact switch date: the second birthday.
The gap between the two ages never changes (it is always the weeks born early), but it matters less as your child grows: 8 weeks is a big share of a 6-month-old's life and a small share of a 20-month-old's.
Corrected age and the CDC milestone checklists
CDC's checklists list what most children (75% or more) can do by each age. They are for monitoring, not a screening test or a diagnosis. CDC suggests using corrected age for a child born more than 3 weeks early, and between two checklist ages it says to use the earlier checklist. The milestones by month pages on this site follow both rules and pair each milestone with everyday activities.
- CDC 12-month checklist, with activities that practise each item
- CDC 15-month checklist, with activities that practise each item
- CDC 18-month checklist, with activities that practise each item
- CDC 24-month checklist, with activities that practise each item
- CDC 30-month checklist, with activities that practise each item
- CDC 36-month checklist, with activities that practise each item
When to talk to your pediatrician
Corrected age helps you read timing; it is not a reason to dismiss a worry. If your child loses a skill they had, is not doing what the checklist for their corrected age describes, or something else concerns you, raise it with your pediatrician. In the U.S., you can contact your state's Early Intervention program for a child under 3 directly, without a doctor's referral.
Tables by weeks born early
Each page has a month-by-month table of corrected and chronological age up to 24 months and the checklist for each month.
- 3 weeks born at 37
- 4 weeks born at 36
- 5 weeks born at 35
- 6 weeks born at 34
- 7 weeks born at 33
- 8 weeks born at 32
- 9 weeks born at 31
- 10 weeks born at 30
- 11 weeks born at 29
- 12 weeks born at 28
- 13 weeks born at 27
- 14 weeks born at 26
- 15 weeks born at 25
- 16 weeks born at 24
Frequently asked questions
How do I calculate corrected age?
Take the age since birth (chronological age) and subtract the number of weeks your baby was born early. Weeks early is 40 minus the weeks of pregnancy at birth, so a baby born at 33 weeks is 7 weeks early, and at 8 months old is about 6½ months corrected. Counting from the due date gives the same answer.
Until what age should I use corrected age?
Until the second birthday. The American Academy of Pediatrics recommends corrected age for judging development during the first two years; from 24 months chronological age on, use the age since birth.
Which CDC milestone checklist should I use for a preemie?
The one for the corrected age. If the corrected age falls between two checklist ages, CDC says to use the earlier checklist, so a child who is 17 months corrected uses the 15-month checklist until 18 months corrected.
Does a baby born 3 weeks early need a corrected age?
CDC suggests correcting when a child was born more than 3 weeks early. At 3 weeks the difference is under a month and matters little after the first year; ask your pediatrician which age they use at checkups.
Can corrected age explain every delay?
No. Corrected age helps you read timing, but it is not a reason to wait on a worry. If your child loses a skill, misses milestones for their corrected age or something concerns you, talk to your pediatrician. In the U.S. you can also contact your state Early Intervention program for children under 3 directly.
Is my child’s birth date sent anywhere?
No. The calculator runs in your browser and nothing you type is sent or saved.
Sources
- AAP HealthyChildren.org: Corrected Age for Preemies
- AAP HealthyChildren.org: Your Preemie's Growth and Developmental Milestones
- CDC “Learn the Signs. Act Early.” milestones
- CDC milestone checklists by age
- CDC free child development resources
Corrected-age rules from the AAP; checklist rules from CDC. How this site uses its sources: sources and method.