If your baby arrived early, you have probably already had the disorienting experience of opening a milestone checklist and finding your child "behind" on almost everything. It is one of the cruellest small experiences of preemie parenthood — and it is usually a measurement error, not a developmental one.
The fix has a name: adjusted age, also called corrected age. Here is what it is, how to work it out, and why it changes the way you should read every chart and checklist you meet in the first two years.
What is adjusted age?
Adjusted age is your baby's age counted from their original due date rather than their birthday. It answers a different question from the one on the birth certificate: not "how long has this baby been alive?" but "how far along is this baby's development?"
A baby born at 32 weeks was due eight weeks later. At four months old on the calendar, that baby has only had about two months of post-due-date development. Comparing them to a typical four-month-old is comparing two children at genuinely different stages, and the checklist that results tells you nothing useful.
How to calculate adjusted age
The arithmetic is simple:
- Start with chronological age — how long since the birthday, in weeks or months.
- Work out how early the baby was: 40 weeks minus the gestational age at birth.
- Subtract the second number from the first.
Adjusted age = chronological age − weeks born early
A worked example. A baby born at 30 weeks was 10 weeks early. At six months old (about 26 weeks), the adjusted age is 26 − 10 = 16 weeks, or roughly four months adjusted. When you read a milestone list for that baby, four months is the row to look at — not six.
| Born at | Weeks early | At 6 months old, adjusted age is |
|---|---|---|
| 37 weeks | 3 | ~5 months |
| 34 weeks | 6 | ~4.5 months |
| 32 weeks | 8 | ~4 months |
| 28 weeks | 12 | ~3 months |
Doing this by hand every single time is exactly the sort of small, repeated, error-prone task that software should absorb. Wobble does it automatically: enter the due date once, and every milestone range and growth chart in the app is corrected from then on. You never do the subtraction again, and you never see your child measured against a due date they never had.
Where adjusted age matters most
Milestones. Rolling, sitting, babbling, walking — these track developmental time, not calendar time. A preemie meeting milestones on their adjusted schedule is, in the ordinary case, doing exactly what you would hope.
Growth charts. Plotting a preemie's weight against their chronological age will drag them down the percentiles for reasons that have nothing to do with how they are feeding. Corrected plotting is what makes the curve readable. (We wrote a whole separate guide to reading those curves: what your baby's growth percentile actually means.)
Sleep and feeding expectations. Sleep consolidation and feeding volumes follow developmental maturity too. "Should be sleeping through by now" is a sentence that deserves correcting before you take it to heart.
Vaccinations are the exception. Immunisations are almost always scheduled by chronological age, not adjusted — a preemie generally gets their shots on the ordinary calendar schedule. This is one of the few places where the birthday, not the due date, is the right number. Your pediatrician will confirm the schedule for your child.
How long do you keep adjusting?
The usual guidance is to correct until around two years old, by which point most children born preterm have caught up enough that the difference stops being meaningful. Some clinicians keep correcting a little longer for babies born very early, or for specific measurements like head circumference.
The honest answer is that the cut-off is a convention rather than a hard line, and your pediatrician's view of your particular child beats any general rule you read online — including this one.
The emotional part nobody puts in the guide
Adjusted age is not a consolation prize or a way of moving the goalposts. It is the more accurate measurement. Using it is not being generous to your baby; it is being correct about them.
That framing matters more than it sounds, because the alternative — a checklist quietly telling you every week that your child is failing — does real damage to how a parent experiences the first year. This is why we designed Wobble's milestones as gentle age ranges rather than deadlines, and why adjusted age is automatic rather than a setting buried three menus deep. The app should not be the thing that makes you anxious at 3am.
Tracking a preemie without the extra maths
If you want the short version of what to look for in a tracker when your baby was born early:
- Automatic adjusted age, applied to milestones and growth charts, not just displayed as a number somewhere.
- Milestones as ranges, so a normal spread of development does not read as failure.
- Head circumference tracking alongside weight and height, since it is watched especially closely in preterm babies.
- Somewhere to keep the medical history — vaccines, first foods, teeth — because preemie parents field more questions from more clinicians than most.
- Data that stays on your phone, since a preterm baby's early health record is about as sensitive as personal data gets.
Wobble was built to tick all five. It is free for one or two children, it needs no account, and nothing about your baby leaves your device. If you are still comparing options, our rundown of the parenting apps worth your time puts it in context.