How do I work out my baby's age in weeks?

Count the days since birth and divide by 7. The whole number is the weeks and the remainder is the extra days, so 51 days is 7 weeks and 2 days. The calculator does this for you and also shows the age in months and days.

When should I switch from weeks to months?

There is no rule, but most parents and health professionals talk in weeks until about 3 months and in months until the second birthday. After that, years and months are easier to follow.

What is corrected age?

Corrected age, also called adjusted age, is the age your baby would be if they had been born on their due date. You take the age from birth and subtract the time they arrived early. A 4-month-old born 8 weeks early has a corrected age of about 2 months.

Until what age is corrected age used?

Usually until about the second birthday. By then most children born early have caught up with children born at term, so their age from birth is used again. For babies born very early, some doctors keep an eye on it for a little longer.

Are vaccinations given by corrected age?

No. Babies born early are vaccinated at the same age from birth as other babies, not by corrected age. Your vaccination schedule follows the date of birth.

My baby was born on the 31st. When are they one month old?

In a month without a 31st, the last day of the month counts. A baby born on 31 January is one month old on 28 February (29 February in a leap year) and two months old on 31 March.

Baby age calculator: weeks, months and corrected age

How the age is counted

The calculator counts calendar time from the date of birth to today, the way you would on a calendar on the kitchen wall. A baby reaches a new month on the same day number of the next month: born on 10 May, they are one month old on 10 June and two months old on 10 July.

Months are not all the same length, so a few dates need a rule. When the next month is too short, the last day of that month counts. A baby born on 31 January turns one month old on 28 February (or 29 February in a leap year) and two months old on 31 March. A baby born on 29 February has their first birthday on 28 February the following year.

Weeks and days are simpler: the calculator counts the days since birth and splits them into whole weeks and the days left over. Day 0 is the day of birth itself.

Why the units change as your baby grows

Nobody says their toddler is 70 weeks old, and nobody says their newborn is 0.3 months old. The calculator shows the age the way people usually say it:

First two weeks: in days, for example 10 days.
Until 3 months: in weeks and days, for example 7 weeks and 2 days.
Until the first birthday: in months and weeks, for example 5 months and 1 week.
From 1 year: in years and months, for example 1 year and 3 months.

Below the headline you also see the exact age in months and days, in weeks and days, and in total days, plus the date of the next month or birthday. That last line is handy if you take a photo every month.

Corrected age for babies born early

A baby born before 37 weeks of pregnancy is called premature, or preterm. For these babies, doctors and nurses often use two ages:

Chronological age: the time since birth, the age on the birth certificate.
Corrected age: the chronological age minus the time the baby was born early, which is the same as the age counted from the due date.

The reason is development. A baby born 8 weeks early has had 8 weeks less time in the womb, so at 4 months from birth they are often doing what a 2-month-old does. Comparing them with their corrected age gives a fairer picture of how they are getting on with smiling, sitting or crawling, and of how they are growing.

To see it, tick the box and add the original due date. The calculator works out how early your baby arrived, the weeks of pregnancy at birth, and the corrected age today.

A worked example

Sam was born on 1 May. Their due date was 20 June, so they arrived 50 days early: 7 weeks and 1 day. A pregnancy is counted as 40 weeks, so Sam was born at 32 weeks and 6 days.

On 14 October, Sam is 166 days old: 5 months and 1 week from birth. Counted from the due date, 20 June, Sam is 3 months and 3 weeks old. That is the corrected age. If Sam is not rolling over yet, that is something many babies of 3 to 4 months are still working on.

Before the due date

If today is still before the due date, there is no corrected age yet. Neonatal teams then usually talk about corrected gestational age: the weeks of pregnancy at birth plus the time since. A baby born at 30 weeks who is now 6 weeks old has a corrected gestational age of 36 weeks. The calculator shows this instead until the due date arrives.

When to stop using corrected age

Most children born early catch up with their peers during the first two years. That is why corrected age is commonly used for milestones and growth until about the second birthday, after which the age from birth is used again. For babies born very early, some doctors keep using it a little longer.

Corrected age is not used for everything. Vaccinations, for example, are given at the same age from birth as for any other baby.

Limits of this calculator

The calculator only does date arithmetic: it cannot tell you whether your baby's development is on track. Babies reach milestones over a wide range of ages, whether they were born early or not. If you have questions about your baby's growth or development, your doctor, nurse or health visitor knows your child and can look at the whole picture.