How corrected age is calculated
The method is the same one used in neonatal follow-up: count the weeks your baby was born early, then subtract them from the age counted from birth.
Weeks early = 40 weeks − gestational age at birth
Estimated due date = birth date + weeks early
Corrected age = today − estimated due date
Chronological age = today − birth date
Corrected age in months = corrected age in days ÷ 30.4375
- 40 weeks is 280 days, the standard length of a pregnancy counted from the last period.
- Days are included. A baby born at 34 weeks 4 days is 37 days early, not 42, so a partial week is never rounded away.
- Months use an average length of 30.4375 days (365.25 ÷ 12). That keeps the figure comparable between months rather than switching length at each calendar month. The weeks-and-days figure is the exact one.
- Babies born at 37 weeks or later are not corrected. From 37 weeks the birth is counted as term, so corrected age equals chronological age and the page says so.
- Corrected age can be below zero. If your baby is not yet at their due date, the corrected age is negative, which is expected; the page shows it rather than hiding it.
- All date maths uses whole UTC calendar days, so a clock change can never shift a result by a day.
Why corrected age is used until 24 months
A baby born early has had less time in the womb, so comparing them with a baby of the same chronological age can make their development look behind when it is following the expected course for their due date. Counting from the due date lines the two up.
The American Academy of Pediatrics, through healthychildren.org, explains corrected age as age counted from the due date and advises using it when assessing development in the first two years. This calculator therefore reports the date on which corrected age reaches 24 months, so you can see where that convention ends.
Correction is a way of making a fair comparison, not a target. If your baby was born at term, there is nothing to correct, and the page does not apply a correction just because the fields are filled in.
Vaccinations and check-ups use chronological age
Corrected age is for developmental assessment. It does not move appointments, and it does not change the vaccination schedule.
Immunisations are given by your baby's actual date of birth. The NHS vaccination schedule and AAP guidance both use chronological age, so a premature baby should not have vaccinations delayed because of the correction. The same applies to the routine baby checks booked from the birth date.
If a clinician asks for the corrected age, give them the birth date and the gestational age at birth as well; they can then apply their own local convention. Do not skip or postpone an appointment because a corrected age makes your baby look younger.
When to ask for advice
Contact your health visitor, paediatrician or family doctor if you are worried about your baby's movement, feeding, growth or development, whatever the corrected age says. A calculator can line up two dates; it cannot examine a baby.
If your baby was born before 37 weeks, follow the follow-up schedule your neonatal unit gave you rather than creating one from this page.
Not medical advice. Early Pregnancy Tools is an educational calculator. It does not assess development, growth or health, and it does not replace the appointments your baby is offered.
Frequently asked questions
What is corrected age?
Corrected age is your baby's age counted from their estimated due date rather than from their birth date. It is calculated by taking the number of weeks early your baby was born away from their chronological age. A baby born 6 weeks early who is now 12 weeks old has a corrected age of 6 weeks.
When do I stop using corrected age?
The American Academy of Pediatrics advises using corrected age for developmental assessment until about 24 months. After that, the difference between corrected and chronological age is small enough that chronological age is used. This page shows the calendar date on which corrected age reaches 24 months so you can see the end point.
Do vaccinations use corrected age?
No. Immunisations are scheduled by your baby's actual date of birth, not by the corrected age. The NHS vaccination schedule and AAP guidance both use chronological age, so a premature baby should not have vaccinations delayed because of the correction.
My baby was born at 37 weeks — do I need a corrected age?
No. From 37 weeks the birth is counted as term, so there is no correction to apply and corrected age is the same as chronological age. This calculator says so explicitly rather than showing a small artificial adjustment.
Sources and disclaimer
Sources used for the method and the notes on this page:
This page is informational only and is not a medical device. It does not create a clinician–patient relationship and must not be used to make medical decisions about your baby's health or development.
Your dates are processed only in this browser tab. Nothing is uploaded, stored or shared.
Related tools
The baby sleep schedule gives sleep ranges by age, which are usually described by chronological age. The pregnancy week tracker is useful if you are planning a next pregnancy and want to record each week.