Adjusted Age Calculator
Work out your baby's adjusted (corrected) age. Enter the birth date and how many weeks and days of gestation your baby was born at; you get corrected age in weeks and months, chronological age, and how early the birth was.
Adjusted age = chronological age minus the weeks born before 40 weeks. Pediatricians use it for milestones and growth charts until about age 2 (AAP guidance).
Why preemies get two ages
A baby born at 32 weeks arrived 8 weeks before the standard 40-week term. Development doesn't skip ahead to match the birth certificate — brain and motor maturation track time since conception, not time since delivery. So pediatricians subtract the missed weeks: a 6-month-old born 2 months early is developmentally about a 4-month-old, and that is the age to use when judging rolling, sitting, babbling, and growth-chart percentiles.
The American Academy of Pediatrics recommends correcting until about 24 months, by which point most of the gap has closed. Vaccines are the big exception — they follow chronological age on the normal schedule regardless of prematurity.
How it’s calculated
Weeks premature = 40 weeks (280 days) − gestational age at birth. Adjusted age = chronological age (days since birth) − days premature, shown in weeks + days and in average months (1 month = 30.4375 days). Babies born at 40+ weeks get no correction. Correction is conventionally applied through 24 months.
This is a convention for tracking development, not medical advice — due-date dating itself carries a week or more of uncertainty, and every preemie develops on their own curve; raise milestone concerns with your pediatrician regardless of what the corrected math says.
How much to correct, by gestational age at birth
| Born at | Weeks early | 6-month-old's adjusted age |
|---|---|---|
| 40 weeks (term) | 0 | 6 months |
| 36 weeks | 4 | ≈ 5 months |
| 32 weeks | 8 | ≈ 4 months |
| 28 weeks | 12 | ≈ 3 months |
| 24 weeks | 16 | ≈ 2 months |
Computed as chronological age minus weeks born before 40; correction used through about age 2 per AAP guidance.
Common mistakes
- Judging milestones by chronological age — a 32-weeker 'behind' at 6 months may be exactly on track at their adjusted 4 months.
- Correcting vaccine timing; immunizations follow chronological age on the standard schedule.
- Counting from the due date instead of subtracting missed weeks — it works out the same only if you do one or the other, not both.
- Still correcting at age 3 or 4 — the convention fades out around 24 months, when the developmental gap has largely closed.
Frequently asked questions
How do you calculate adjusted age?
Adjusted age = chronological age − weeks premature, where weeks premature = 40 − gestational age at birth. A baby born at 32 weeks who is now 25 weeks old has an adjusted age of 25 − 8 = 17 weeks, about 4 months.
How long should we use adjusted age?
Until about 24 months, per common AAP guidance. After age 2 the developmental difference has usually narrowed enough that everyone switches to regular chronological age.
Does adjusted age apply to growth charts?
Yes — plot weight, length, and head circumference at the corrected age (or use dedicated preterm charts like Fenton early on). Otherwise a healthy preemie looks falsely below the curve.
Do vaccines follow adjusted or chronological age?
Chronological. Preterm babies get immunizations on the same schedule as term babies, counted from the actual birth date — one of the few places the correction is deliberately ignored.
My baby is behind even by adjusted age — what should we do?
Bring it to your pediatrician; that is exactly the comparison the correction exists to sharpen. Preemies also qualify for early-intervention evaluations in most US states, and earlier support works better.