Tools · Baby
Corrected Age Calculator for Premature Babies
Enter your baby's birth date and original due date to get corrected (adjusted) age, gestational age at birth, and how many weeks early they arrived.
The 40-week date you were given before your baby arrived.
Enter both dates to see corrected age.
What corrected age is, and why it exists
A baby born ten weeks early has spent ten fewer weeks growing than a baby born on their due date. Measured from birth, the two babies are the same age. Measured from where their development actually started, they are not. Corrected age — also called adjusted age — closes that gap by counting from the original due date instead of the birth date.
This matters most in the first two years, when milestones arrive quickly and a two-month difference is a large share of a baby's whole life. A baby born at 30 weeks who is six months old chronologically has a corrected age of about three and a half months. Expecting them to sit unsupported like a six-month-old sets up worry over a baby who is developing exactly on schedule for their stage.
The calculator also shows gestational age at birth — how many weeks pregnant you were when your baby arrived. The World Health Organization groups preterm birth as extremely preterm (before 28 weeks), very preterm (28 to 32 weeks), and moderate to late preterm (32 to 37 weeks). Clinicians often use this grouping when discussing what to expect, so it is a useful number to have alongside the dates.
Which age gets used where
The two ages are not interchangeable, and mixing them up is the most common source of confusion for parents of preterm babies. As a general rule, corrected age is used for growth charts, developmental milestones, and feeding progression — anything measuring how far along your baby is. Chronological age is used for vaccinations and for the calendar date of routine check-ups, which are counted from birth regardless of how early your baby arrived.
If a chart or a milestone list ever makes your baby look behind, the first question to ask is which age it was plotted against. A preterm baby plotted at chronological age will look like they are trailing when nothing is wrong. Your clinic can tell you which they are using, and it is a reasonable thing to ask at every appointment.
Corrected age is usually set aside somewhere around two years, by which point most children born preterm have caught up. That is a general pattern rather than a fixed rule — how early your baby was born changes it, and your provider decides when to stop correcting. Everything here is calculated on your device; no dates are stored or uploaded.
Frequently Asked Questions
How do I calculate corrected age by hand?
Subtract the weeks of prematurity from your baby's chronological age. First find how early your baby arrived: count the weeks between the birth date and the original due date. Then subtract that from the age since birth. A baby born 7 weeks early who is 20 weeks old has a corrected age of 13 weeks. The simplest shortcut is to count forward from the due date instead of the birth date — the result is the same.
Is corrected age used for vaccinations?
No. Immunisations are scheduled by chronological age — the time since birth — not corrected age. A baby born two months early still receives their first vaccines on the same calendar schedule as a baby born at term. Corrected age is used for growth and developmental expectations, not for vaccine timing. Follow the schedule your provider gives you.
My baby was born at 35 weeks. Do I still need corrected age?
Babies born between 34 and 37 weeks are described as late preterm, and the difference corrected age makes is small but real — around one month at 35 weeks. Many clinicians still correct for it during the first year, particularly when reviewing motor milestones. Ask your provider whether they are plotting your baby's chart by corrected or chronological age, since practice varies.
Should growth charts be plotted at corrected age?
Yes — for babies born preterm, weight, length, and head circumference are normally plotted against corrected age rather than chronological age. Plotting against chronological age makes a healthy preterm baby appear to be falling behind when they are growing exactly as expected for their stage. Your clinic will tell you which age they are using.
When will my premature baby catch up?
Most children born preterm have caught up with their peers by around 2 years of corrected age, which is why corrected age is usually set aside at that point. Babies born extremely early may take longer, and catch-up does not happen evenly across every area at once — a baby may catch up in growth before motor skills, or the reverse. Your provider will tell you when to stop correcting.
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