Pregnancy Weeks to Months: Understanding the Math Behind Your Bump

Before getting pregnant myself, I’d written quite a bit about the journey to conception and knew that doctors typically count pregnancy in weeks. So when I got pregnant myself and people would ask how far along I was, I’d say something like “Oh, I’m 24 weeks today!” That sort of response was often met with a confused look. But figuring out my pregnancy in months involved some serious mental gymnastics. (“I think that means I’m six months along—or is it seven?”)

Pregnancy math—calculating where you are in weeks, months, and trimesters—can be confusing, even for those of us who are already relatively fluent in bump speak. Here’s why—along with a quick cheat sheet to help you break down exactly how far along you are at any given moment.

Key Takeaways

• Pregnancy is actually between 9 and 10 months long. A full-term pregnancy lasts 40 weeks (or roughly 280 days), which is somewhere between 9 and 10 months long since most months don’t have 4 weeks exactly.
• Doctors count in weeks for a reason. Tracking pregnancy by weeks allows for more precise monitoring of fetal growth and developmental milestones, which is why your OB will always reference your week number rather than your month.
• The math gets tricky because of how pregnancy is dated. Your 40-week countdown officially starts on the first day of your last menstrual period—before you’ve even conceived—although most people don’t find out they’re pregnant until at least week 5.

Pregnancy Weeks to Months Chart

Confused about where exactly you are in your pregnancy? Here’s a chart that helps define timeline by trimester, month and weeks.

How Many Weeks Are in a Pregnancy?

There are officially 40 weeks in pregnancy. That said, you’re officially considered full-term in pregnancy at the beginning of week 39 through the end of week 40. And doctors may allow a pregnancy to continue until the end of week 41 before scheduling an induction.

How many weeks are in a trimester?

Each trimester of pregnancy lasts 13 full weeks, which breaks down as follows:

First trimester: Week 1 (the first day of your last period) to the end of week 13 (13 weeks, 6 days) Second trimester: Week 14 through the end of week 27 (27 weeks, 6 days) Third trimester: Week 28 through delivery

Why Calculating Pregnancy Weeks to Months Is Tricky

Although many of us tend to think of our pregnancies in terms of months, doctors count in weeks to better track fetal growth and milestones, and offer more precise and standardized care.

Still, even providers know this can be confusing for pregnant patients and families. Dr. Tracy Bohn Hemmerdinger, MD, chief of obstetrics and gynecology at NewYork-Presbyterian Hudson Valley Hospital, acknowledges this truth: “That’s something I don’t think that people have come around to yet. They still think in months,” she says.

And the math can get complicated—especially since most people only realize they’re pregnant after a missed period. If this is the case, they’re technically finding out during the second month of pregnancy (or when they’re at least 5 weeks pregnant).

Doctors calculate pregnancy and due date beginning at the first day of your last menstrual period. While it’s hard to pinpoint exactly when ovulation and conception happened, most people do know the date of their last period, says Dr. Tanise Branche, MD, the medical director of labor and delivery at Temple Health and an associate professor at the Lewis Katz School of Medicine at Temple University. Bottom line: pregnancy “begins” approximately two weeks before you’ve even conceived. Confusing, right?

Additionally, there’s no easy way to translate weeks into months, since a month is 30 or 31 days (not counting February)—so it doesn’t break down into neat seven-day increments.

“Weeks to months is confusing, but I really think it’s a nice courtesy to people to tell them how far along you are in months, not weeks. No one speaks the language of pregnancy weeks unless they recently had a baby,” says The Bump community member DarcyLongfellow.

How to Calculate Pregnancy Weeks to Months

To calculate weeks into months, take the number of weeks pregnant you are and divide by four. If you’re counting by months, multiply by four for a rough idea of your pregnancy week.

How many weeks is 1 month pregnant?

The first month of pregnancy could be weeks 1, 2, 3 and 4.

How many weeks is 2 months pregnant?

If you’re calling yourself “2 months pregnant,” it might mean you’re in your 5th, 6th, 7th or 8th week of pregnancy.

How many weeks is 3 months pregnant?

At 3 months pregnant, you could be in weeks 9, 10, 11, 12 or 13. (Remember months aren’t actually four weeks exactly.)

How many weeks is 4 months pregnant?

Your doctor won’t ever really say you’re 4 months pregnant. Instead, they’ll tell you you’re 14, 15 16 or 17 weeks pregnant.

How many weeks is 5 months pregnant?

Five months pregnant isn’t an official milestone. Rather, you’re either 18, 19, 20, 21 or 22 weeks pregnant.

How many weeks is 6 months pregnant?

At approximately 6 months pregnant, you’re either in week 23, 24, 25, 26 or 27.

How many weeks is 7 months pregnant?

If you’re 7 months pregnant, you’re in week 28, 29, 30 or 31.

How many weeks is 8 months pregnant?

At 8 months pregnant, you’re either 32, 33, 34 or 35 weeks pregnant

How many weeks is 9 months pregnant?

Thought pregnancy wrapped up after nine long months? Sorry to say, you have a few more weeks to go. You’re either 36, 37, 38, 39 or 40 weeks along.

Here’s a quick-reference pregnancy chart in months that breaks it all down:

Month 1: week 1, week 2, week 3, week 4
Month 2: week 5, week 6, week 7, week 8
Month 3: week 9, week 10, week 11, week 12, week 13
Month 4: week 14, week 15, week 16, week 17
Month 5: week 18, week 19, week 20, week 21, week 22
Month 6: week 23,week 24, week 25, week 26, week 27
Month 7: week 28, week 29, week 30, week 31
Month 8: week 32, week 33, week 34, week 35
Month 9: week 36, week 37, week 38, week 39, week 40

It’s not completely even and it’s not perfect math. If you want to be precise, stick to monitoring your pregnancy in weeks.

To Sum It Up

The math lesson learned is that pregnancy doesn’t squarely fit into a standard calendar. But remember, pregnancy lasts a total of 40 weeks—or between 9 and 10 months—beginning on the first day of your last menstrual period. Your provider will keep tabs in weeks, and you probably will too (you’ll get used to it, we promise). And as for anyone else who asks how far along you are, feel free to round up to the nearest month—give or take is good enough!

How We Developed This Article

Calculating pregnancy weeks to months isn’t exactly straightforward, especially given that doctors don’t typically measure the journey this way. To help demystify the math of pregnancy, we interviewed two ob-gyns and consulted websites for institutions such as the American College of Obstetricians and Gynecologists (ACOG). We also scoured The Bump forums to find quotes from parents who were confused about how to pinpoint what month of pregnancy they were in. Finally, this article was fact-checked and then reviewed by our ob-gyn medical adviser. Learn more about how we ensure the accuracy of our content through our editorial and medical review process.

About the author Colleen de Bellefonds started writing about pregnancy well before she became a mom herself—but, when she did become pregnant, she still was perplexed about pregnancy weeks-to-months conversion. Luckily, she was able to tap the experts in her network for answers. Colleen is a Paris-based science journalist and a mother of two. (Don’t ask her what week or month of parenthood she’s in though!)

Please note: The Bump and the materials and information it contains are not intended to, and do not constitute, medical or other health advice or diagnosis and should not be used as such. You should always consult with a qualified physician or health professional about your specific circumstances.