How adjusted age is calculated
A baby born early has two important dates: the day they were born and the day they were due. Adjusted age, also called corrected age, counts from the due date. The AAP explains the method in two steps:
- Subtract your baby's gestational age at birth from 40 weeks. That's how many weeks early they arrived.
- Subtract those weeks from your baby's actual age, the time since their birth date.
| Born at | Weeks early | Actual age | Adjusted age |
|---|---|---|---|
| 32 weeks | 8 weeks | 16 weeks (4 months) | 8 weeks (2 months) |
| 28 weeks | 12 weeks | 26 weeks (about 6 months) | 14 weeks (about 3 months) |
| 35 weeks | 5 weeks | 52 weeks (12 months) | 47 weeks (about 11 months) |
The calculator does the same thing in days, so a birth at 33 weeks 4 days counts as 6 weeks 3 days early.
Why adjusted age matters
Development follows time since conception more closely than time since birth, especially early on. The AAP's example: a baby born 8 weeks early who is now 4 months old may just be starting to hold their head up and smile, which is right on track for a 2-month-old. Comparing them with other 4-month-olds, who may be starting to roll over, would make them look behind when they aren't. During the first 2 years, the AAP says corrected age gives a better idea of when a premature baby should reach common developmental goals.
When to use adjusted age, and when not to
| For | Use |
|---|---|
| Milestones (smiling, rolling, sitting, first words) | Adjusted age, during the first 2 years |
| Growth charts | Ask your pediatrician which age they plot |
| Vaccines | Actual age from birth (CDC) |
| Babies born at 37 weeks or later | Actual age: they aren't premature |
Premature birth in numbers
The CDC defines preterm birth as birth before 37 completed weeks of pregnancy. In 2022 it affected about 1 in every 10 babies born in the United States (10.4%). If your baby is one of them, the questions in this calculator are ones your NICU team and pediatrician answer every day, so bring them to your next visit.
To see how your baby compares on the growth charts, use our baby percentile calculator with your baby's due date in place of the birth date.