Baby age calculator
Corrected age is the standard basis for assessing development in babies born preterm, normally until about two years. Vaccination schedules follow chronological age. Follow the advice of your health visitor or paediatrician.
Baby age is usually given in weeks for the first few months and in months thereafter. For a baby born preterm — premature, in the word most parents are given first — corrected age subtracts the weeks born early: a baby born at 34 weeks was six weeks early, so at 8 weeks old the corrected age is 2 weeks. Both are shown together, on any date.
How to work out baby age
Corrected age exists because development runs from conception, not from birth. A baby born ten weeks early has had ten fewer weeks to grow, and expecting them to sit, smile or crawl on the chronological schedule sets an unfair benchmark that reads alarmingly and means very little. The American Academy of Pediatrics set out the terms in 2004: chronological age is time since birth, corrected age subtracts the weeks born before term, and the two are not interchangeable.
Clinicians use corrected age for milestones until around two years, by which point most of the gap has closed, and usually for growth charts over the same period. Chronological age stays the right figure for vaccination, because immune response follows time since birth and not developmental stage. That split is why both numbers sit on screen together: the question decides which one you read, and neither replaces the other.
The arithmetic is small. Forty weeks minus the gestational age at birth gives the weeks born early, and that comes off the age since birth. A baby born at 32 weeks was eight weeks early, so at five months old the corrected age is three months, and a three-month checklist is the fair one to hold them against.
What people use it for
- Filling in a developmental checklist
- Comparing a preterm baby against milestones fairly
- Understanding a paediatric assessment
- Plotting growth on the right chart
- Working out age in weeks for a health visitor
- Tracking feeding or sleep guidance by age
Questions
Chronological age minus the number of weeks born before 40 weeks. It is the fair basis for comparing development in preterm babies.
Forty weeks minus the gestational age at birth. A baby born at 34 weeks was six weeks early.
Usually until about two years, by which point most of the gap has closed.
Yes. The two names describe the same calculation, and which one you hear depends on where you are being seen. Some units say “age corrected for prematurity”, which is the same figure again.
No — vaccination schedules follow chronological age from birth, because immune response tracks time since birth. Ask your health visitor if unsure.
Usually yes, for the first couple of years. Your health visitor will say which chart they are plotting on.
Birth before 37 completed weeks, by the WHO definition. Anything from 37 to 42 weeks is term, and no correction is applied to it.
Take the age since birth and subtract the weeks born early. A baby born at 32 weeks was eight weeks early, so at 20 weeks old the corrected age is 12 weeks.
It is on the discharge summary and in whatever record your health service keeps for the baby. Without it no correction can be applied, and the plain age is what you have.
No. Corrected age is only ever subtracted, so leave the weeks-early field at zero.
Roughly 4.35, which is why weeks and months diverge. This tool shows both.
Because early development moves quickly and weeks give the resolution that months would blur.
Yes. The second date is the one the age is measured to, so you can look back at a past appointment or forward to a future one.