How to work it out

Corrected age — also called adjusted age or corrected gestational age — is your baby's age counted from your original due date instead of the day they were born. The arithmetic is one subtraction. Take your baby's actual age in weeks, then subtract the number of weeks they were born early.

The American Academy of Pediatrics works the second number out as 40 minus the gestational age at birth. Their worked example: a baby born at 32 weeks was eight weeks early. If she is now four months old, which is 16 weeks since birth, her corrected age is eight weeks, or two months. So you should expect the developmental skills of a two-month-old, not a four-month-old. Rolling front to back, which term babies often manage around four months, would be unrealistic. Holding her head up and smiling is exactly right.

Once your baby is past their due date the same sum works in months: actual age in months minus months born early. The HSE gives the example of a 14-month-old born two months early having a corrected age of 12 months.

What NICE says about the two-year rule

This is not folk wisdom. NICE guideline NG72 tells health professionals to "explain to parents or carers that their child's developmental (corrected) age, which is calculated from their original due date (and not the date they were born), will be used for the first 2 years when assessing their functional and developmental skills (such as walking and talking)." At every follow-up visit and assessment, professionals are told to "correct for gestational age up to 2 years when assessing development".

Bliss puts the same point in a sentence worth repeating to relatives: "Premature baby's development should always be assessed by their due date and not the date they were actually born."

Two years is a rule of thumb, not a switch that flips. The gap between corrected and actual age matters enormously at three months and barely at all at three years, because the same number of weeks is a shrinking fraction of your child's life. For a baby born at 24 weeks, four months of correction is most of their first year; for a baby born at 35 weeks it is five weeks, and it will have washed out long before the two-year mark.

What corrected age is used for

Use it for anything that is about your baby's development and growth:

Milestones — smiling, head control, rolling, sitting, crawling, walking, first words.
Plotting on growth charts. UK preterm babies are plotted with correction for gestation, which is why the chart your health visitor uses may look different from the one your friend has.
Readiness for solid food, which is judged on corrected age and on the physical signs rather than the birth certificate.
Milk feeding. The HSE advises offering breast milk or formula as the main milk drink until 12 months corrected age, and using first infant formula rather than follow-on milk up to that point.
Sleep expectations, which are developmental like everything else.

Vaccinations run on actual age

This is the exception that catches people out, and it is important. Routine childhood immunisations are given according to actual age from the date of birth, not corrected age. A baby born at 28 weeks has their eight-week vaccinations at eight weeks old, often while still on the neonatal unit. Preterm babies are more vulnerable to the infections these vaccines prevent, not less, so delaying is the wrong instinct. If your baby is still an inpatient, the unit gives them.

Health visitor reviews and the NHS development review schedule also run on actual age as appointments, even though what is assessed at them is judged on corrected age. Bring both numbers.

Growth charts, and why yours looks different

Growth is where correction becomes visible. In the UK, babies born preterm are plotted with their gestational age taken into account, and very preterm babies are usually plotted on a chart designed for neonatal and infant close monitoring rather than the standard chart in the red book. If you are handed a chart that looks unfamiliar, that is why.

The practical consequence is that a preterm baby's dot sits where a baby of their corrected age should sit, not where a baby of their birth date should. A nine-month-old born at 30 weeks plotted uncorrected looks alarmingly small; plotted corrected, they may be sitting comfortably on the fiftieth centile. Head circumference is corrected too, and is watched closely because head growth tracks brain growth.

Whichever chart is used, the shape of the line matters more than any single point. A baby tracking steadily along a low centile is usually less interesting to a clinician than a baby crossing centiles downwards, corrected or not.

If you are not sure of the due date

Corrected age depends on knowing the expected date of delivery, which is normally the one set by the dating scan rather than by your last period. If your pregnancy was dated late, was an IVF pregnancy, or was a twin pregnancy where the dates were revised, use the EDD written on your maternity notes and neonatal discharge summary. That is the number the follow-up team will be using, and it is worth all of you working from the same one.

When people give you the wrong age

Every preterm parent gets this: a relative, a nursery, sometimes a locum GP, comparing your baby against their chronological age and finding them behind. Bliss's advice is practical — "always remind them of your baby's corrected age". A sentence works better than a chart: "She's seven months old but she was three months early, so she's four months corrected." Say it before the assessment, not after.

After two years

Most guidance stops correcting at two years, and NICE's own follow-up assessment at four years is explicitly at uncorrected age. That does not mean prematurity stops mattering — it means correction is no longer the right tool for measuring it. If your child is still visibly behind their peers after correction has been dropped, that is a reason to ask for an assessment, not a reason to keep subtracting. Talk to your GP, health visitor or the neonatal follow-up team rather than doing the arithmetic yourself.

Corrected Age and Why It Matters