Pregnancy Weeks to Months: Your Calculator

Pregnancy weeks to months—the kind of word problem you hope never comes up on a math test.

The doctor measures it in weeks. Your family wants to know the number of months.

And there you are—wondering how far along you really are.

Look, you’ve got a lot on your plate right now.

Between navigating pregnancy symptoms and getting life ready for your new arrival, the last thing you need is a weekly brain teaser.

Plus, pregnancy brain fog is a thing.

As this recent study examined, pregnancy actually changes your brain physiologically.

So if you’re struggling to do the math right now, you’re more than excused.

Don’t worry, we’re here to help.

Here’s your guide to converting pregnancy weeks to months.

In this article: 📝

How do you calculate weeks into months in pregnancy?

First things first, when does your pregnancy even start? Here’s the deal:

Healthcare providers measure your pregnancy in weeks, with Day 1 being the first day of your last menstrual cycle (LMP).

This can be confusing, because getting your period is pretty much the opposite of being pregnant.

Your fertile window is actually about two weeks after your period starts, around the time of ovulation, and that’s likely when conception took place.

But because ovulation (and conception) can be tricky to pinpoint, doctors calculate from a point that is easier to lock down—and that’s your LMP.

So essentially, by calculating the length of your pregnancy from that first day of your LMP, you add approximately two weeks to your IRL pregnancy.

So now that you know what Day 1 is, where do you go from there?

Your pregnancy calendar

If you ask most people how long a human pregnancy is, they will probably give you the answer nine months.

And they would be about right.

Ask them how many weeks in a pregnancy and they might say 40.

And this would also be about right.

The problem is no two pregnancies are alike, and babies don’t like sticking to exact schedules.

So what does that mean for your due date?

How do we begin to calculate this?

A well-known method of calculation goes all the way back to an obstetrician called Franz Naegele, who was at work in the early 1800s.

According to Naegele’s rule—something we very much still use today—your EDD (or Estimated Due Date) is calculated as follows:

• Start from the first day of your LMP
• Count back three calendar months from that date.
• Add one year and seven days to that.
  And that adds up to about forty weeks of pregnancy.

But this is by no means an exact science—and Naegele’s rule has been found to be a little inaccurate.

The more up-to-date way to get your due date is through an ultrasound—but even this cannot predict your due date with accuracy.

Even more accurate is if your menstrual dating aligns with dates based on a first-trimester ultrasound.

In fact, it appears that due dates are inaccurate--meaning not the actual date your baby will arrive—about 96% of the time.

The reality is, if your baby is born somewhere between 37 and 42 weeks, they’re on time.

What about the different definitions of pregnancy term?

Well, the American College of Obstetricians and Gynecologists (ACOG) class them like this:

• Before 37 weeks is considered preterm (or premature).
• From the beginning of the 37th to the end of the 38th week is considered early-term.
• From the beginning of the 39th to the end of the 40th week is considered full-term.
• Between the beginning and end of the 41st week is considered late-term.
• After 42 weeks is considered post-term.

So how do you know how far along you are at any given point?

If we can all agree that the weeks do not fit neatly into the months, here’s your cheat sheet for calculating weeks of pregnancy to months pregnant.

How many weeks pregnant is 1 month?

Weeks: 1 - 4 Trimester: First

Find out more about what to expect at 1 month pregnant

How many weeks pregnant is 2 months?

Weeks: 5 - 8 Trimester: First

Find out more about what to expect at 2 months pregnant

How many weeks pregnant is 3 months?

Weeks: 9 - 12 Trimester: First

Find out more about what to expect at 3 months pregnant

How many weeks pregnant is 4 months?

Weeks: 13 - 16 Trimester: Second

Find out more about what to expect at 4 months pregnant

How many weeks pregnant is 5 months?

Weeks: 17 - 20 Trimester: Second

Find out more about what to expect at 5 months pregnant

How many weeks pregnant is 6 months?

Weeks: 21 - 24 Trimester: Second

Find out more about what to expect at 6 months pregnant

How many weeks pregnant makes 7 months?

Weeks: 25 - 28 Trimester: Third

Find out more about what to expect at 7 months pregnant

How many weeks is 8 months pregnant?

Weeks: 29 - 32 Trimester: Third

Find out more about what to expect at 8 months pregnant

How many weeks is 9 months pregnant?

Weeks: 33 - 36 Trimester: Third

Find out more about what to expect at 9 months pregnant

How many weeks is 10 months pregnant?

(That's right, technically, you're 10 months pregnant at this stage!)

Weeks: 37 - 40 Trimester: Third

Pregnancy weeks to months chart

Fancy something to print out so you can keep track?

Well, consider this our gift to you, mama-to-be:

No more mental gymnastics, just refer to this whenever you're trying to figure out your pregnancy weeks to months.

Exciting times!

Good luck, mama-to-be!