How age and corrected age are worked out
Age is the time from the birth date to the date you count to: whole days, weeks and days, and calendar months and days. A baby born on the 31st is one month old on the last day of a shorter month. For corrected age, the AAP says to begin with the baby’s age in weeks and subtract the weeks the baby was preterm [1]. Counting from the due date gives the same answer.
Corrected age = age − (due date − birth date) = time since the due date
Used for babies born before 37 weeks [2], during the first 2 years [1].
Example: a baby born at term
Born Sun, Mar 15, 2026, counted to Mon, Nov 2, 2026: 232 days = 33 weeks 1 day, or 7 months 18 days.
Example: a baby born early
Born Mon, Jan 5, 2026, due Mon, Mar 2, 2026 (8 weeks early, at 32 weeks), counted to Mon, Apr 27, 2026.
- Age: 16 weeks.
- Corrected age: 16 − 8 = 8 weeks, about 2 months, matching the AAP’s example of a 32-week baby at 16 weeks.
This is date arithmetic. For questions about your baby’s growth or development, ask your child’s doctor.
Questions
What is corrected age?
For a baby born early, the age counted from the due date instead of the birth date. The AAP’s method: take the baby’s age in weeks and subtract the number of weeks the baby was preterm [1].
When should corrected age be used?
The AAP says that during the first 2 years, corrected age gives a better idea when a baby should reach common developmental goals [1]. This calculator shows it only while the baby is under 24 months.
Who counts as preterm?
The NICHD calls birth before 37 weeks preterm [2]. The due date is 40 weeks, so a baby born more than three weeks before it gets a corrected age here. A baby born later than that is not corrected.
Why are months and weeks both shown?
Months are calendar months, so their length varies. Weeks are exact. Both are shown because charts and clinics use either.
Is a baby who is behind on a milestone a concern?
This page cannot tell you that. Ask your child’s doctor, who can look at your baby’s whole picture.