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Corrected Age Calculator for Premature Babies (Adjusted Age)

Work out a premature baby’s corrected (adjusted) age and chronological age in months, weeks and days from the birth date and gestation or the due date, with postmenstrual age, the prematurity category, the date to stop correcting (UK RCPCH rule or age 2 or 3) and a table of corrected-age milestone dates.

Date of birth

I know the

Gestation at birth — weeks

plus days

Original due date

Age on date

Correct until

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How it works

weeks early = 40 weeks − gestation at birth; corrected age = chronological age − weeks early (equivalently, age counted from the due date); postmenstrual age = gestation at birth + chronological age

A baby born ten weeks early has had ten weeks less time to develop than a term baby with the same birthday, so milestones and growth are judged against the age they would be if they had been born on their due date. That is the corrected (or adjusted) age: chronological age minus the number of weeks born before 40 weeks, which is the same as counting age from the original due date. This calculator gives both ages in calendar months and days and in weeks and days, the postmenstrual age (gestation at birth plus time since birth) used by neonatal units before and around the due date, and the WHO prematurity category: extremely preterm under 28 weeks, very preterm 28 to 31 weeks and moderate to late preterm 32 to 36 weeks. How long to correct depends on the service. UK RCPCH growth chart guidance corrects until 1 year for babies born at 32 to 36 weeks and until 2 years for babies born before 32 weeks, and never for babies born at 37 weeks or later. Many US clinics correct until the second birthday for every preterm baby, and some follow-up programmes continue to age 3 for the earliest babies; pick the rule under More options. Enter the gestation at birth or the original due date. Vaccinations always follow chronological age. The table shows the date your baby reaches each corrected age, which is handy for planning developmental checks and the start of solid foods.

Worked example

Born March 10, 2026 at 30 weeks 4 days (214 days): 280 − 214 = 66 days early, so the due date was May 15, 2026. On October 24, 2026 the baby is 228 days old (7 months 14 days, or 32 weeks 4 days) chronologically, but 228 − 66 = 162 days corrected: 5 months 9 days, or 23 weeks 1 day. Born before 32 weeks, the RCPCH rule says to correct until the 2nd birthday, March 10, 2028. The baby reaches 6 months corrected on November 15, 2026.

Sources: RCPCH: Plotting preterm infants (UK-WHO growth chart factsheet) · WHO fact sheet: Preterm birth (categories by gestational age) · RCPCH UK-WHO growth charts: guidance for health professionals

Frequently asked questions

How do I calculate corrected age?

Subtract the number of weeks your baby was born early (40 weeks minus the gestation at birth) from their actual age. A 20-week-old baby born at 32 weeks is 20 − 8 = 12 weeks corrected. Counting age from the due date gives the same answer.

Which age do vaccines follow?

Chronological age. Immunisations are given at the actual age since birth, not the corrected age, even for very premature babies.

When do we stop using corrected age?

UK RCPCH guidance corrects until 1 year for babies born at 32 to 36 weeks and until 2 years for babies born before 32 weeks. Many US clinics correct until 2 years for all preterm babies, and some programmes until 3 years for babies born before 28 weeks. By then most babies have caught up.

Should I use corrected age for starting solids?

Usually yes: most services use corrected age, aiming for around 6 months corrected and looking for readiness signs, though very premature babies may get individual advice. Ask your baby’s team.

What is postmenstrual age?

Gestational age at birth plus the time since birth. A baby born at 30 weeks who is 4 weeks old has a postmenstrual age of 34 weeks. Neonatal units use it before and around the due date.

Do babies born at 37 to 39 weeks need corrected age?

No. Babies born at 37 weeks or later are full term, and their actual age is used for growth charts and milestones.

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