You will get corrected and chronological age — for example: 3 mo 2 w instead of 5 mo.
A baby born early develops on their own timeline: weight, length and skills are compared not with peers of the same birth date but with children of the same corrected age — adjusted for the weeks missed before the due date. This calculator works out both the chronological and the corrected age.
How corrected age is calculated
- Degree of prematurity = estimated due date − date of birth. For example, a baby born on 1 April with an EDD of 1 June was 8 weeks 5 days early.
- Corrected age = chronological age − degree of prematurity. If that baby is now 6 months old since birth, the corrected age is about 4 months.
- Gestational age at birth = 40 weeks − prematurity. This is what the discharge notes call “born at 31 weeks”.
Weight, length and skills should be compared against the corrected age — otherwise it looks as if the child is “behind”, when in fact they’re developing exactly on their own schedule.
Worked example
| Value | Result |
|---|---|
| Date of birth | 1 April |
| Estimated due date | 1 June |
| Degree of prematurity | 8 weeks 5 days |
| Gestational age at birth | 31 weeks 2 days |
| Age on 1 October (chronological) | 6 months |
| Corrected age | about 4 months |
FAQ
How long should I correct for?
Usually until age 2: by then most children born prematurely have caught up with their peers and the correction is dropped. For very preterm babies a doctor may keep applying it longer.
Which age do I use for weight and growth?
The corrected age. WHO growth curves are built for full-term babies, so for a premature child the correct reference point is the corrected age, not the birth date.
Do vaccinations and weaning follow the corrected age too?
No. Vaccinations normally follow the chronological age — the actual date of birth. Weaning timing and other practical questions are decided individually by your paediatrician; there’s no single rule here.
What if I don’t know the due date?
Take the gestational age at birth from the discharge notes: if the baby was born at 32 weeks, prematurity is roughly 8 weeks (40 − 32). Add that to the birth date to get an approximate due date.