What is the difference between chronological and corrected age?

Chronological is simply the time since birth. Corrected subtracts the time they were short of reaching 40 weeks of gestation. A chronologically 6-month-old baby born at 32 weeks has a corrected age of 4 months.

Is age also corrected for vaccines?

No: the vaccination schedule follows chronological age, not corrected age. Correction applies to growth (weight, height, head circumference) and developmental milestones, not to vaccines.

Until what age should correction continue?

As a general guide, up to 2 years of chronological age, and somewhat longer for extremely premature babies (before 28 weeks). Past that age, the difference stops being relevant against normal variation between children.

My baby was born at 38 weeks — do I need to correct their age?

Technically yes, though the difference (2 weeks) is small enough that it rarely changes anything in practice. Correction matters more the more premature the birth.

Corrected age for premature babies

The age to use to properly read their percentiles and milestones

Comparing a baby born at 32 weeks to a chart built for babies born at 40 is not fair: they missed out on two months of development inside the womb. Corrected age exists for exactly that: it subtracts the time they had left to be born, and it is the age to use when reading their growth percentiles and developmental milestones during the first years.

Work out the corrected age

Current age (months)

Weeks of gestation at birth

+ days

Fill in current age and gestational weeks.

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How it is calculated

You subtract from the chronological age (time since birth) the time they were short of reaching 40 weeks of gestation, the length of a full-term pregnancy. A baby born at 32 weeks was 8 weeks short of 40; if they are 5 months old today, their corrected age is 5 months minus 2 months (8 weeks), i.e. 3 months.

A baby born at term (40 weeks or more) needs no correction: their chronological and corrected age are the same, which is why this calculator makes no sense for term births.

Why it actually matters

Uncorrected, a premature baby would systematically compare below where they should on weight, height and milestones like holding up their head, sitting or walking — not because something is wrong, but because they are being compared to babies with more total development time, inside and outside the womb. Corrected age is what allows a fair comparison.

It is the age paediatricians and neonatologists use to interpret growth charts and developmental scales during a preemie's first years, and the one worth keeping in mind when comparing with other babies of the same chronological age.

Until when is it corrected?

The general recommendation — from the American Academy of Pediatrics — is to keep using corrected age up to 2 years of chronological age for physical growth, and up to that same age or somewhat longer (depending on how premature) for developmental milestones. In extremely premature babies (born before 28 weeks), some specialists keep correcting up to age 3.

Beyond that, the difference of a few weeks or months becomes statistically insignificant against the rest of normal variation between children, and correction is dropped.

Catch-up growth

Many preemies, especially those born at an appropriate weight for their gestational age, show a period of accelerated growth — called catch-up growth — during the first 1-2 years, moving closer to the curve they would be on had they been born at term. It is not automatic and does not happen in every case, and it is exactly what a paediatrician watches for at a premature baby's check-ups.

Frequently asked questions

What is the difference between chronological and corrected age?: Chronological is simply the time since birth. Corrected subtracts the time they were short of reaching 40 weeks of gestation. A chronologically 6-month-old baby born at 32 weeks has a corrected age of 4 months.
Is age also corrected for vaccines?: No: the vaccination schedule follows chronological age, not corrected age. Correction applies to growth (weight, height, head circumference) and developmental milestones, not to vaccines.
Until what age should correction continue?: As a general guide, up to 2 years of chronological age, and somewhat longer for extremely premature babies (before 28 weeks). Past that age, the difference stops being relevant against normal variation between children.
My baby was born at 38 weeks — do I need to correct their age?: Technically yes, though the difference (2 weeks) is small enough that it rarely changes anything in practice. Correction matters more the more premature the birth.

Sources

American Academy of Pediatrics — 'Age Terminology During the Perinatal Period'. Pediatrics, 2004 (later reaffirmed).
Official definitions of chronological age, gestational age and corrected age, and the recommendation to correct up to age 2.

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