Chronological age and corrected age
A baby born prematurely has two ages. The chronological age counts from the birth date. The corrected age, or adjusted age, counts from the date the baby was originally due, at 40 weeks of pregnancy. It reflects the baby’s development better during the first two years, since a baby born two months early has had two months less time in the womb.
The formula
- Weeks born early = 40 − weeks of pregnancy at birth
- Corrected age = chronological age − weeks born early
- Original due date = birth date + (280 days − pregnancy length at birth)
Worked example
A baby was born at 32 weeks, 120 days ago.
- Weeks born early: 40 − 32 = 8 weeks.
- Chronological age: 120 days, or 17 weeks.
- Corrected age: 120 − 56 days = 64 days, 9 weeks and 1 day, about 2.1 months.
- The original due date was 56 days after the birth.
When to use the corrected age
| Use | Which age |
|---|---|
| Growth charts | corrected age, up to about 2 years |
| Developmental milestones | corrected age |
| Starting solid foods | usually corrected age, with your pediatrician |
| Vaccinations | chronological age |
| School entry | chronological age |
Most preterm babies catch up with their peers by age 2 to 3. Very premature babies, born before 28 weeks, may need longer follow-up.
Degrees of prematurity
| Category | Weeks at birth |
|---|---|
| Extremely preterm | less than 28 |
| Very preterm | 28 to 31 |
| Moderate to late preterm | 32 to 36 |
| Term | 37 and over |
In the US, about 1 baby in 10 is born preterm.
Milestones by corrected age
A baby born at 32 weeks who is 6 months old has a corrected age of 4 months, so smiling, head control and reaching are compared with 4-month-old babies, not 6-month-olds. Comparing with full-term babies of the same chronological age can cause needless worry.
A note on medical advice
This calculator gives estimates based on averages. It does not replace advice from your doctor, midwife or health care provider, who will date your pregnancy and follow it with exams and ultrasounds.