How Many Weeks, Months, and Trimesters in a Pregnancy?

Your pregnancy countdown can get jumbled pretty quickly. Are you 13 weeks pregnant or 14? First trimester or second? Three months pregnant or four?

Then there’s figuring out your due date — which, for some moms-to-be, is the trickiest formula of all. Trusted SourceAmerican College of Obstetricians and GynecologistsMethods for Estimating the Due DateSee All Sources ^[1] 

“I am so confused about the due date calculation,” says What to Expect Community parent lilamyo228.

Fortunately, the answers are (relatively) simple once you know what to count. Here’s what you need to know about how many weeks, months, and trimesters there are in a pregnancy — and where you fit in.

Key Takeaways

• There are 40 weeks in a pregnancy, but by the time you actually conceive, you’re already in week 3.
• The first trimester of pregnancy lasts until week 13, the second trimester starts at week 14 and ends at week 27, and the third trimester lasts from week 28 to week 40.
• Your due date is calculated from the first day of your last menstrual period.

How many weeks are in a pregnancy?

There are 40 weeks (or 280 days) in a pregnancy. Your due date is calculated starting from the first day of your last menstrual period (even though most pregnancies are not conceived until two weeks later).

Keep in mind that only about 18% of pregnancies will reach precisely week 40, according to data from the Centers for Disease Control and Prevention (CDC). Trusted SourceCenters for Disease Control and PreventionShifts in the Distribution of Births by Gestational Age: United States, 2014-2022See All Sources ^[2]

Track your baby during every week of pregnancy with the What to Expect app, where you can record symptoms in the My Journal tool, keep up with baby's growth and development, and connect with other parents who are due at the same time.

How many weeks are in a trimester?

There are 13 or 14 weeks per trimester. (Just divide 40 by 3, and you’ll see why the breakdown isn’t exact.)

How many trimesters are in a pregnancy?

There are three trimesters in a pregnancy, according to the American College of Obstetricians and Gynecologists (ACOG): Trusted SourceAmerican College of Obstetricians and GynecologistsHow Your Fetus Grows During PregnancySee All Sources ^[3] the first trimester, which lasts from weeks 1 to 13; the second trimester, which lasts from week 14 to week 27; and the third trimester, which lasts from week 28 to week 40 (and up).

Read This Next

Your Guide to the First Trimester of Pregnancy

Your Guide to the Second Trimester of Pregnancy

Your Guide to the Third Trimester of Pregnancy

Your Guide to the First Trimester of Pregnancy

Your Guide to the Second Trimester of Pregnancy

Your Guide to the Third Trimester of Pregnancy

It can be confusing, since we talk about trimesters, but the length of pregnancy isn’t evenly divisible by three.

Dr. Shannon Smith, OB/GYN and Member of the What to Expect Medical Review Board

Is pregnancy nine or 10 months long?

Your 40 weeks of pregnancy are counted as nine months (even though pregnancy is a just little bit longer than nine months). But wait … there are four weeks in a month, which would make 40 weeks 10 months. Right? Not exactly. 

Four weeks is 28 days, but months (with the exception of February, of course) have 30 or 31 days, making each month about 4.3 weeks long. 

“It can be confusing, since we talk about trimesters, but the length of pregnancy isn’t evenly divisible by three,” says Shannon Smith, M.D., an OB/GYN at Brigham Faulkner OB/GYN Associates in Boston, Massachusetts, and a member of the What to Expect Medical Review Board. 

How many weeks is a full-term pregnancy?

If you're carrying a single baby, a full-term pregnancy is between the start of 39 weeks up to the end of 40 weeks. If you're carrying twins, a full-term pregnancy is considered at least 38 weeks.

ACOG uses the following terms for pregnancy, depending on when your baby is born:

• Preterm: 20 weeks to 36 weeks 6 days 
• Early term: 37 weeks to 38 weeks 6 days
• Full term: 39 weeks to 40 weeks 6 days
• Late term: 41 weeks to 41 weeks 6 days
• Post term: after 42 weeks

Pregnancy months to weeks chart

Here’s a simple breakdown of approximately how the weeks, months, and trimesters are counted in pregnancy:

Trimester 1

• Month 1: Weeks 1 to 4
• Month 2: Weeks 5 to 8
• Month 3: Weeks 9 to 13

Trimester 2

• Month 4: Weeks 14 to 17
• Month 5: Weeks 18 to 22
• Month 6: Weeks 23 to 27

Trimester 3

• Month 7: Weeks 28 to 31
• Month 8: Weeks 32 to 35
• Month 9: Weeks 36 to 40

How do I know what week I am currently in my pregnancy?

Your doctor says you're 15 weeks pregnant, but you’re also hearing that you’re in week 16. Which is right? The short answer: both. 

It turns out how many weeks into pregnancy you are is a little different than how many weeks pregnant you are. If you’re in week 16, you’re actually 15 weeks and some days pregnant. Fifteen full weeks have gone by, but not 16 weeks, so people say that you're both 15 weeks pregnant and in week 16 of pregnancy.

Not convinced? Look at it this way: When you turned 1 year old, you had lived through your first year and were starting on your second. In other words, on your first birthday, you were in day one of your second year. 

But no one said you were 2 years old until you'd finished that second year and had begun your third.

The same rules apply when talking about your weeks of pregnancy: You are x weeks pregnant, but in your x+1 week of pregnancy.

How do I calculate my due date?

Your due date is calculated from the first day of your last menstrual period (LMP), not the day you conceived. So try to remember the day your last period started and add 40 weeks to that.

You can also calculate your due date with the following formula:

• Subtract three months from the first day of your last menstrual period
• Add seven days
• Add one year

For example, if the first day of your last menstrual period was June 2, 2025, subtract three months to get March 2, 2025. Add seven days and you'll get March 9, 2025. Finally, add one year to find your estimated due date of March 9, 2026.

Why is pregnancy calculated from your last period?

The pregnancy clock starts even before sperm meets egg (and, actually, before your ovary even dropped the fated egg that made your baby). Using your last period is simply a more reliable benchmark to date a pregnancy from.

Even if you’re a master at tracking your cervical mucus and are sure about the day you ovulated — as well as the day(s) you had sex — you probably can’t pinpoint the exact moment of conception.

That’s because there’s a bigger window than you might think: Sperm can hang out and wait for an egg to fertilize for three to five days after they’ve arrived, and an egg can be fertilized for up to 24 hours after it’s been released. 

“If someone knows their date of conception or date of ovulation, that could be used to narrow their specific due date, but as a general method, the first day of the last menstrual period is the standard for initially marking and communicating the estimated due date,” says Dr. Smith. 

That's also good news, since it means you’ll already have clocked four weeks of pregnancy by the time you miss your period.