The formula
Weeks of prematurity = 40 weeks − gestational age at birth.
Corrected age = chronological age − weeks of prematurity.
Stated in one line: corrected age = chronological age − (40 weeks − gestational age at birth). Forty weeks is 280 days, and it is the reference length of a full-term pregnancy; anything before 37 completed weeks counts as preterm under the World Health Organization's preterm birth definition. The gap between those numbers — 40 weeks minus the gestational age the baby actually reached — is the fixed amount you subtract for the whole of the child's early follow-up. It never changes; only the chronological age keeps moving.
Everything below is worked out in days and converted at the end, which avoids the rounding errors you get from working in whole months.
Worked example 1 — 28 weeks, now 6 months old
- Prematurity: 280 − (28 × 7) = 280 − 196 = 84 days (12 weeks)
- Chronological age: 6 months = about 183 days
- Corrected age: 183 − 84 = 99 days = about 3 months 8 days
This baby's first birthday cake is six months away, but developmentally you should be reading the 4-month milestone row soon, not the 6-month row — and that milestone set itself comes from the 2022 evidence-informed milestones review in Pediatrics.
Worked example 2 — 34+3 weeks, now 10 weeks old
- GA in days: (34 × 7) + 3 = 241
- Prematurity: 280 − 241 = 39 days
- Chronological: 70 days
- Corrected: 70 − 39 = 31 days, about 4½ weeks
Late preterm correction is the one most often skipped. At this age it more than halves the expected developmental age — which is exactly why the late preterm baby guide exists and why stopping too early causes unnecessary referrals.
Worked example 3 — 25 weeks, still before the due date
- Prematurity: 280 − 175 = 105 days
- Chronological: 60 days
- Corrected: 60 − 105 = −45 days, i.e. 6 weeks 3 days before term
- Postmenstrual age: 25 weeks + 60 days = 33+4 weeks PMA
Before term-equivalent age, corrected age is negative and PMA is the number clinicians actually use. The tool shows both. The same two inputs read a discharge note in reverse: a baby born at 29+2 weeks who is 5 weeks old today is 29+2 + 5 weeks = 34+2 weeks PMA, and that is the figure the clinic will quote at that visit.
Worked example 4 — 36 weeks, now 12 weeks old
- GA in days: 36 × 7 = 252
- Prematurity: 280 − 252 = 28 days (4 weeks)
- Chronological: 12 weeks = 84 days
- Corrected: 84 − 28 = 56 days = 8 weeks corrected
A 36-week birth sits in the moderate to late preterm band — the case most often dismissed, because the baby looks term at birth. Twelve weeks after delivery the four-week gap is still a third of the baby's life, so the milestone row you should read is the 2-month one, not the row implied by 12 weeks of calendar age. Skipping this example is how a healthy late-preterm baby gets compared with the wrong calendar.
Common questions about corrected age
Do you correct a premature baby’s vaccine schedule?
No. Immunisations are scheduled by chronological age. Corrected age is used for many developmental conversations, not to delay vaccines.
What if corrected age is negative?
Before the original due date, corrected age can be below zero. The tool displays that value and also shows postmenstrual age, which is the clinically useful age before term-equivalent age.
When should corrected-age calculations stop?
Developmental correction is commonly used until about 24 months, with domain-specific variation. See when to stop correcting for the full explanation.
The mistakes that actually happen
- Using 9 months instead of 40 weeks. Nine calendar months is 273–276 days depending on the months involved. Use 280.
- Using 37 weeks as the reference. 37 weeks is the threshold below which a birth counts as preterm, not the length of a full-term pregnancy. Subtracting 37 instead of 40 under-corrects by three weeks every single time.
- Correcting immunisations. Vaccines are given by chronological age, as the CDC immunization schedule by age makes explicit. Correcting delays protection in the most vulnerable infants.
- Swapping PMA for corrected age. PMA starts at gestational age at birth and only ever rises; corrected age starts negative and reaches zero at the due date. They answer different questions — see the PMA calculator for the distinction.
- Rounding gestational age down to whole weeks. 34+6 is nearly a week different from 34+0.
- Forgetting to recalculate. The gap in weeks is fixed; the proportion of the child's life it represents shrinks every month, which is exactly why correction stops being used later on.
A quick check on your own answer: corrected age must always be smaller than chronological age for a baby born early, and the difference between the two must equal exactly (40 weeks − gestational age at birth) for ever. If a clinic's number differs from yours by more than a few days, the usual causes are a rounded gestational age, a due date recalculated from an early scan, or arithmetic done in months instead of days — all three are worth asking about rather than assuming the tool is wrong.
Related preterm follow-up guides
Use the corrected age calculator for a date-specific result, compare adjusted age vs chronological age, review a typical NICU follow-up schedule, read the late preterm baby guide, review red flags, or inspect the methodology and sources.