A baby's age is counted from the date of birth in weeks for the first couple of months, then in months. For a baby born early, the corrected (or adjusted) age is the age minus the weeks they arrived before their due date — the same as counting from the due date. A baby born at 32 weeks on June 1, 2026 is 21 weeks old on October 30, 2026, but has a corrected age of 13 weeks and 4 days.
Enter the birth date — and, for a premature baby, the weeks of pregnancy at birth — to get both ages, the due date, and how long to keep correcting.
Worked example
Baby's date of birth: June 1, 2026Age on: October 30, 2026Born at (weeks of pregnancy): 32 weeksand days: +0 days
Baby's age on Oct 30, 2026: 4 months, 29 days
Born 8 weeks early, so the corrected age is 21 weeks 4 days minus 8 weeks: 13 weeks 4 days, or 3 months and 3 days counted from the due date of July 27, 2026.
Open this example in the calculator →How to use the Baby Age Calculator
- Enter your baby's date of birth.
- Leave "Age on" as today, or choose the date of a check-up.
- If your baby was born early or late, choose the weeks (and days) of pregnancy at birth. Leave it at 40 weeks for a baby born at term.
- Read the age in weeks, months and days, and the corrected age for a baby born early.
How to work out corrected age
A full-term pregnancy is counted as 40 weeks. Subtract the gestational age at birth from 40 to get the weeks your baby was born early, then subtract that from their age:
- Born at 32 weeks: 40 − 32 = 8 weeks early.
- Age on October 30, 2026, from a birth on June 1: 21 weeks and 4 days.
- Corrected age: 21 weeks 4 days − 8 weeks = 13 weeks and 4 days (about 3 months).
The same answer comes from counting the age from the due date — here July 27, 2026 — which is how the calculator does it.
When to use corrected age
Corrected age is for growth and development: comparing a baby with growth charts and with milestones such as smiling, sitting, crawling and first words. A baby born two months early is not expected to sit at the same chronological age as a baby born at term. Vaccinations follow the chronological age — the actual birth date — in almost every schedule.
Most clinicians stop correcting at about 2 years, by which time most preterm children have caught up. For babies born very early — before 28 weeks — some recommend correcting until 3. Your child's doctor or health visitor will say which to use.
Before the due date: postmenstrual age
For a baby still in the neonatal unit before their due date, the corrected age would be negative. Neonatal teams use the postmenstrual age (PMA) instead: the gestational age at birth plus the time since. A baby born at 28 weeks who is 6 weeks old is "34 weeks PMA". The calculator shows it until the due date.
Weeks, months, years
By convention a baby's age is given in weeks until about 8 weeks, in months until 2 years, and in years and months after that. A month birthday is the same day of each month as the birth date.
Frequently asked questions
How do I calculate my baby's corrected age?
Subtract the number of weeks your baby was born early (40 minus the weeks of pregnancy at birth) from their age since birth. A baby born at 32 weeks who is 6 months old has a corrected age of about 4 months.
Until what age should I use corrected age?
Usually until about 2 years. For babies born before 28 weeks some clinicians correct until 3. Ask your child's doctor.
Do premature babies get vaccines by corrected age?
No. Vaccination schedules follow the actual date of birth (chronological age), not the corrected age.
How many weeks old is my baby?
Enter the birth date and the calculator shows the age in weeks and days. A baby born on September 1 is 8 weeks and 4 days old on October 31.
What is full term?
Babies born between 39 and 40 weeks 6 days are full term; 37 to 38 weeks 6 days is early term; before 37 weeks is preterm.