Pregnancy math can get confusing pretty quickly. Are you in week 12 or 13? Your first trimester or your second? Three months pregnant or four?

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.

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 30 percent of pregnancies will reach precisely week 40.

How many weeks are in a trimester?

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

How many trimesters are in a pregnancy?

There are three trimesters in a pregnancy: the first trimester, which lasts from approximately weeks 1 to 13, the second trimester, which lasts from about week 14 to week 27, and the third trimester, which lasts from around week 28 to week 40 (and up).

Is pregnancy nine or 10 months long?

Your 40 weeks of pregnancy are counted as 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.

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.

The American College of Obstetricians and Gynecologists (ACOG) as outlined the following terms for pregnancy, depending on when your baby is born:

  • Preterm: 20 weeks to 37 weeks 
  • Early term: 37 weeks 0 days to 38 weeks 6 days
  • Full term: 39 weeks 0 days to 40 weeks 6 days
  • Late term: 41 weeks 0 days to 41 weeks 6 days
  • Post term: after 42 weeks 0 days

Pregnancy months to weeks chart

If these explanations have your head spinning, 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 to Calculate Your Due Date

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

Your doctor says you're 15 weeks pregnant, but you’re also being told that you’re in week 16. Which is right? Short answer: both. In both cases, your due date is the same — it’s the language referring to it that’s different.

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.

To clear things up, think about birthdays. When you were born, you weren’t even a day old but you were in your first week of life. 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?

Still haven’t checked in with the doc about your estimated due date? It’s calculated counting from the first day of your last menstrual period (LMP) instead of the day you conceived. So try to remember the first day of your LMP and add 40 weeks to that.

If you like math, you can also calculate your due date with the following formula:

  • Subtract three months from the first day of your LMP
  • Add seven days
  • Add one year

For example, if the first day of your LMP was 6/2/2020, subtract three months to get 3/2/2020. Add seven days and you'll get 3/9/2020. Finally, add one year to find your estimated due date of 3/9/2021.

Why is pregnancy calculated from your last period?

Why start the clock on pregnancy before sperm even meets egg (and, actually, before your ovary even dropped the fated egg that made your baby)? The LMP is simply a more reliable day to date a pregnancy from. Even if you’re a master of 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. That's also good news, since it means you’ll already have clocked four weeks of pregnancy by the time you miss your period.