Your baby’s age in days, weeks and months
We count the days since the birth, then show them as weeks and days, and as months and days.
Why we count month birthdays
Parents usually think in “month birthdays”: a baby born on 10 May is 1 month old on 10 June. Months have different lengths, so we count calendar months, not an average.
Corrected age for babies born early
For a baby born before 37 weeks, doctors often use “corrected age”: the age minus the number of weeks the baby was born early.
A worked example
A baby born at 32 weeks is 8 weeks early (40 − 32). At 4 months old, the corrected age is about 2 months.
How long corrected age is used
Corrected age is used to check growth and development, usually during the first 2 years. Your baby’s doctor will tell you which age to use.
Questions people ask
How do I work out my baby’s age in weeks?
Count the days since the birth and divide by 7. The calculator does this for you and also shows days and months.
What is corrected age?
It is your baby’s age minus the weeks they were born early, counted from 40 weeks. A baby born at 32 weeks who is 16 weeks old has a corrected age of 8 weeks (AAP).
Until what age is corrected age used?
Usually during the first 2 years (AAP). Your baby’s doctor will tell you when to stop.
Which age should I use for milestones?
For a baby born early, the corrected age gives a better idea of when to expect milestones (AAP). Ask your baby’s doctor if you are unsure.
Is a baby born at 37 weeks premature?
No. Babies born before 37 completed weeks of pregnancy are called preterm, or premature (WHO).
Do the vaccine dates use the corrected age?
Our vaccine dates count from the birth date. If your baby was born early, ask your baby’s doctor or nurse which dates apply to your baby.
Why do months and weeks not match?
Months have different lengths, so 3 months is not always 13 weeks. We count calendar months from the birth date.
Read the full guide: Baby age calculator
Sources
- HealthyChildren.org (American Academy of Pediatrics): Corrected age for preemies · checked October 26, 2026
- WHO: Preterm birth fact sheet · checked October 26, 2026