How To Calculate Baby’s Age

How To Calculate Baby’s Age. How to know how many weeks translate to the number of months old a baby is and if you count from due date or birth date.

Home holding baby

Many new moms find calculating the age of your baby slightly confusing. Your friend’s baby is 16 weeks old, while your cousin’s is 4 months old, yet that isn’t the same age. This post is just to quickly clarify that for anyone who needs to know.

You can count your baby’s age by days, weeks, and months. At some point, you move on to years, though I still didn’t say years with Brayden when he was almost three.

There are not only 4 weeks in a month (except February), so every 4 weeks of your baby’s life does not equal one month. Here is a quick chart reference:

How to calculate baby's age
  • 4 Weeks = a couple of days shy of 1 month (it will depend on which month your child was born in)
  • 8 Weeks = almost a week shy of 2 months
  • 12 Weeks = one week shy of 3 months
  • 13 Weeks = 3 Months old
  • 16 Weeks = about a week and a half shy of 4 months old
  • 17.5 Weeks = 4 months old
  • 20 Weeks = about 2 weeks shy of 5 months old
  • 22 Weeks = 5 months old
  • 24 Weeks = 2-2.5 weeks shy of 6 months old
  • 26 Weeks = 6 months old

You can see from this list that 4 weeks does not equal one month. Both ages can indicate developmental milestones for you, so be sure you are comparing correct ages when looking at developmental milestones as well as Babywise milestones.

You want to move baby’s age up on the same day of the week he was born. So if your baby was born on a Thursday, then every Thursday he will get one week older.

The months, however, will move to the next month on the date of the birth even if the day of the week is not the same. So your baby born on Saturday the 14th will turn one month old on Tuesday the 14th even though it is a Tuesday.

If you want to make this easier on yourself to calculate the age of a person, this free age calculator will do the math and calculate your age in years, months, weeks, days, hours, minutes, and seconds. It doesn’t get much more precise than that.

Gestational Age or Birth Day

People often wonder if they should count how old baby is based on what gestational age said baby would be born (due date) or if you should count from baby’s actual birth date.

In other words, do we go by baby’s due date or by baby’s birth date?

When you are saying how old your baby is, you would start with 8 weeks. But many moms will say, “My baby is 8 weeks. 4 weeks adjusted.”

This means that the baby was born 4 weeks before the due date.

This age difference can seem weird to point out for those who haven’t had a baby born early. The birth date is a clear indicator for how old baby is. Birthdays don’t lie.

However, when you are considering how long to expect baby to be sleeping, how often baby should be eating, the milestones baby should be reaching, and which Wonder Weeks mom should be consulting, adjusted age can play a big role.

A general rule of thumb is if your baby was born premature, you consider the adjusted age of your baby when you do the calculation of how old baby is.

All three of my girls were born 3 weeks early.

Something I always tell people when I get asked the question “how old is my baby who was born early” is that they need to observe baby and see where baby seems to be.

A birth date calculated off of the due date is only as accurate as the due date was.

Due dates can be wrong.

Gestational age (how far along you are in pregnancy) is calculated based on the first day of your last menstrual period (LMP).

But that doesn’t necessarily tell you for sure when the conception date was.

Some women have 28 day cycles. Some have 30 day cycles. Some vary month to month. Most women do not know exactly when ovulation happened. So the first day of your last period last month might have been 29 days before the next cycle starts while mine might be 32 days.

I had two different doctors for my five pregnancies. One did Brayden, Braxston, and Kaitlyn. Then he moved and I had a different one for McKenna and Brinley.

My first doctor moved my due date back by one week with every pregnancy. My second doctor never did that.

My three girls were all born 3 weeks early.

So, was Kaitlyn really 2 weeks early? Or was McKenna really 4 weeks early?

The point is, due dates are estimates and not an exact science.

If your baby was born within one week of the due date, I would just go by birth date. I would probably do the same with up to two weeks early. I would do the same with a baby born late.

If your baby is 4 weeks early or earlier, I would do some adjusting of the age.

I find 3 weeks this in between. Observe your baby and see when the milestones are met and when wonder weeks seem to hit. You might find your baby, who was born 3 weeks early, seems to behave just 2 weeks younger than she is, not 3.

Look at all types of milestones. Look at the physical milestones (when does she roll over? When is that first social smile?) as well as schedule milestones (when can she sleep longer stretches? When can she go longer between feedings?) and the milestones you see with the Wonder Weeks.

Newborn baby sleeping wearing a crocheted bunny costume

What Age Do You Tell People

So at what ages do you give people when they ask?

I think that depends on your audience. When Brayden was a baby, I kept track of his weeks until he was a year old, then I moved to months until he was two. Then I did just turned two, almost 2.5, 2.5, close to three, almost three.

Once he hit three, I did number of years. But anyone who interacts with children knows that you always add in the half. A child is not simply 6 years old. He is “almost 6.5” or “6.5” or “almost 7.”

With Kaitlyn, I made things easier on myself. I kept track of weeks until she was about 4 months old, then I did months. I am moved to years as soon as she hit one year old.

If someone who doesn’t have young children asks how old my baby is, I do months. Weeks usually mean nothing to them. If it is a nurse or a doctor, I do weeks up to 4-6-ish months. If it is another mother who also has young children, I do weeks up to 3-4 months then do months.

My reasoning behind this is to give the person the answer they are looking for.

A childless person usually don’t know how to equate months with weeks, and before I had kids, I always wondered why my friend wouldn’t just tell me how many months her child was instead of weeks.

A doctor or nurse knows what needs to be happening with your child down to the week, so that is why I do weeks much longer. A week really can make a difference in the accomplishments of your child.

A baby who is 12 weeks old will have different milestones met than one who is three-months-old.

Another mother with young children has a point of reference with weeks, so that is why I do that with her for a while. That is what I do–but you do what you want.

How to calculate babys age

Reader Comments

  • brisnicki said…
    This is a funny post that I can _so_ relate to :)My DD will be 7 months next week. I stopped counting weeks at around the 3 to 4 month mark as I kept losing count of the week. She was born right at the beginning of the month so it was much easier to look at current date and work out her age from that.I remember a friend once telling me her daughter was now 27 months old.. I paused to count how many years that was when she explained ‘2 years and 3 months’. Oh of course… I would have got there eventually, I am just slow at my 12 times tables :)Child ages are similar to pregnancy gestation. As mother I counted off the weeks one at a time, but while ’20 weeks’ meant a lot to me (first ultrasound) it did not mean much to the lay person. They’d rather hear it in months so they know how far you’re along based on ‘9 months’ 🙂

    The Babywise Mom said…
    So true Brinsnicki 🙂

Reader Questions

  • bradysmom said…
    So is 28 weeks really 7 months old?

    The Babywise Mom said…
    No, that would still be 6 months old.

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How To Calculate Baby's Age. How to know how many weeks translate to the number of months old a baby is.
Calculate baby's age graphic

This post originally appeared on this blog May 2008

4 thoughts on “How To Calculate Baby’s Age”

  1. Hello, I was induced at 37 weeks due to a medical condition and my baby was born at a healthy weight (3.47 kg and 50cm height). He is currently 11 weeks old (8 adjusted), should I go by his adjusted age or by his birth date?

    • The tricky thing is that a due date isn’t necessarily spot on. So if you go by due date, you can be wrong. My 3 girls were all born at 37 weeks. I found they each had their own timetable in reaching milestones and the wonder weeks, so I would just see what has been typical and go off of that.

  2. So confusing lol and this is my 4th child. I don’t remember how I counted my other children’s ages. My baby was born October 31,2022. So she is not 3 months until tomorrow which is January 30,2023

    • She would be 3 months on January 31. But the crazy thing about a 31st birthday is the months without 31 days…do you go 30? 1? And February…You also have the half birthday of April, with no 31.


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