Adjusted age calculator for premature babies
Adjusted age is your baby's age counted from their due date instead of their birthday. Enter the date of birth and the gestational age at birth and this tool gives you the adjusted age, the actual age, and the gap between them. It works entirely in your browser.
Adjusted age calculator for premature babies
No sign-up · PrivateThe day your baby was actually born.
Completed weeks. It is on the discharge summary, and the neonatal team will have said it out loud more times than you can count.
The "and 4 days" part of "30 and 4".
Leave blank for today. Fill it in if you are preparing for an appointment on a particular date.
Adjusted (corrected) age
as at 1 July 2026
Actual (chronological) age: 0 days
The difference: 10 weeks — that is how much of the calendar to set aside
Due date (adjusted age counts from here): 9 September 2026
Born at: 30 weeks (preterm)
Nothing you type leaves your device. The whole calculation runs in your browser.
How this is calculated
FormulaHow adjusted age is calculated
Adjusted age, also called corrected age, is your baby's age counted from their due date rather than their birthday. The calculation has four steps and you can do it on paper:
- Weeks early = 40 weeks 0 days − the gestational age at birth. A baby born at 30 weeks 0 days was 10 weeks early. A baby born at 24 weeks 0 days was 16 weeks early.
- Due date = date of birth + weeks early.
- Actual (chronological) age = the calendar age from the date of birth to today.
- Adjusted (corrected) age = the calendar age from the due date to today.
The difference between the two is always the number of weeks early, and it never changes as your baby grows. A baby born at 30 weeks who is 12 weeks old has an adjusted age of 2 weeks. A baby born at 32 weeks who is 16 weeks old has an adjusted age of 8 weeks, which is roughly 2 months — this is the worked example the American Academy of Pediatrics uses.
Correction is applied only to babies born before 37 completed weeks, which is the definition of preterm used by the World Health Organization, the NHS and the RCPCH. For a baby born at 37 weeks or later, the standard milestone charts already assume a term birth, so adjusted age and actual age are the same number and this calculator says so rather than manufacturing a difference.
Before the due date has been reached, adjusted age is negative and clinicians use corrected gestational age instead: gestational age at birth + weeks since birth. A baby born at 27 weeks 2 days who is 4 weeks old is about 31 weeks 3 days corrected gestation.
How long correction is applied: the American Academy of Pediatrics advises using corrected age during the first 2 years. The RCPCH UK-WHO growth charts use a narrower convention for plotting — correct until 1 year for babies born at 32 to 36 weeks, and until 2 years for babies born before 32 weeks. Around two years is the usual answer either way.
The calculator uses true calendar months, not 30-day blocks, so its answer matches the one written in a red book or a clinic note. It also reports the age in weeks, because that is the language neonatal follow-up appointments are conducted in.
Why adjusted age matters more than the calendar
If your baby was born prematurely, you have probably had this conversation. Someone asks how old the baby is. You say the number of months since the birth. They say something about what their own child was doing at that age, and you go home wondering whether yours is behind.
They are measuring against the wrong yardstick, and so, quite often, is everybody else. A baby born at 28 weeks has spent three months of their life doing developmental work that a term baby did inside the uterus. The milestone charts — rolling over, sitting, babbling, walking — were built from babies born at term. Comparing a preterm baby's calendar age against them does not measure the baby. It measures the prematurity, over and over again.
Adjusted age fixes that. It is your baby's age counted from the day they were due rather than the day they arrived, and it is the number your neonatal team, health visitor and paediatrician use when they assess development.
Working it out
Subtract the gestational age at birth from 40 weeks to get how early your baby was. Add that to the birthday and you have the due date. Adjusted age is simply the age counted from there.
Bliss, the UK charity for babies born premature or sick, gives this example: a baby born at 30 weeks gestation, so 10 weeks early, who is now 12 weeks old has an uncorrected age of 12 weeks and a corrected age of 2 weeks. The American Academy of Pediatrics gives a similar one: a baby born at 32 weeks is 8 weeks premature, so at 4 months old (16 weeks) the corrected age is 2 months. As the AAP puts it, “even if your baby is 4 months old, you should expect her to have the developmental skills of a 2-month-old term baby.”
That sentence is worth reading twice, because it is the whole point. Nothing is wrong. The expectation was wrong.
How long do you keep correcting?
This is the question parents ask most, and it has two defensible answers depending on which side of the Atlantic you are on.
The American Academy of Pediatrics states that “during the first 2 years, using your baby's corrected age will give you a better idea when he or she should reach common developmental goals.” Two years is the widely used figure, and by then many babies born preterm have closed the gap.
The RCPCH UK-WHO growth charts use a narrower convention for plotting growth: correct for gestational age until 1 year for babies born at 32 to 36 weeks, and until 2 years for babies born before 32 weeks. The RCPCH is candid that part of the reason for the cut-off is practical — correcting beyond those ages is difficult on a paper chart, and the impact of correction shrinks as a child gets older.
Neither is a deadline. Correction is a convention for fair comparison, not a milestone your child has to hit. Some babies catch up well before two; some, particularly those born very preterm or with additional medical needs, take longer, and that is information for your follow-up team rather than a verdict.
Where adjusted age is used — and where it is not
Use adjusted age for developmental milestones, growth chart plotting, and conversations about what your baby “should” be doing. Weaning is usually discussed in terms of corrected age too, though the guidance for preterm babies is individualised and Bliss and your dietitian are better guides than a general rule.
Immunisations are the important exception. Preterm babies are vaccinated according to their actual chronological age, on the same schedule as everyone else, and should not be delayed. Babies born very prematurely are more vulnerable to the infections vaccines prevent, not less. If anyone suggests waiting until your baby “catches up”, that is a conversation to have with your GP or neonatal team before anything is postponed.
Saying the number out loud
The practical value of knowing the adjusted age is that it gives you something to say. Health visitors, nursery staff, relatives and well-meaning strangers will reach for the birthday, because that is the number they have. “She is six months old, but four months corrected” ends the comparison before it starts, and it usually shifts the conversation from concern to interest.
Bring it to appointments. A premature baby's development should be assessed against their due date, and if a professional is working from the birthday, saying the corrected age is the fastest way to fix it.
What this calculator does not do
This is arithmetic. It is not a developmental assessment, and it cannot tell you whether your baby is doing well. Babies born preterm vary enormously, catch-up is rarely a straight line, and a number on a screen is no substitute for the people who know your baby.
If you are worried — about feeding, movement, hearing, vision, or anything that has changed — your health visitor, GP or neonatal follow-up team is the right place to take it. Take the adjusted age with you. It makes the conversation start from the right place.
Sources
- Corrected Age For Preemies — American Academy of Pediatrics (HealthyChildren.org), accessed
- UK-WHO growth charts, 0-4 years, and Plotting Preterm Infant Growth (Fact Sheet) — Royal College of Paediatrics and Child Health, accessed
- Early developmental milestones — Bliss, accessed
- RCPCH Digital Growth Charts: date and age calculations (gestational age correction) — Royal College of Paediatrics and Child Health, accessed