Pregnancy in weeks, months, and trimesters

How long is pregnancy? The short answer is 280 days, 40 weeks, 3 trimesters, or a little more than 9 months.

pregnant woman sitting on an exam table speaking to a physician who is making notes on a chart
Photo credit: / PeopleImages

How many weeks pregnant am I?

To determine your due date, healthcare providers count 40 weeks from the first day of your last menstrual period (LMP) rather than trying to guess exactly when sperm met egg. Visit our Pregnancy Due Date Calculator to check how many weeks pregnant you are.  

Healthcare providers use your LMP to date pregnancy because many women don't know which day they ovulated. And even if you know the day you had sex that resulted in pregnancy, that may not be the day you conceived: Sperm can linger in your uterus for up to five days waiting for an egg to be released so they can fertilize it.

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Using your LMP to find your due date means that when you get confirmation that you're pregnant, you'll most likely be 4 or 5 weeks along. 

How many weeks are in a pregnancy?

There are 40 weeks in a pregnancy – though you're just as likely to deliver your baby a few weeks before or after that.

How many weeks are in a trimester?

Each trimester is 13 or 14 weeks long. The first trimester lasts until you're 13 weeks pregnant, the second trimester spans week 14 to week 27, and the third trimester starts the day you turn 28 weeks pregnant and lasts until week 40 (or until you deliver your baby).

How many trimesters are in a pregnancy?

There are three trimesters: the first trimester (early pregnancy), the second trimester (mid-pregnancy), and third trimester (late pregnancy).

Pregnancy weeks to month chart

Check out our chart to see how the weeks, months, and trimesters of pregnancy line up with each other.

illustrated chart detailing how to count your pregnancy in trimesters months and weeks

How many months pregnant am I?

You can use the chart above to figure out how the weeks of pregnancy correspond to months. Note that you're not technically one month pregnant until after 4 weeks have passed, for example. But you are "in your first month" during the first 4 weeks and "in month nine" during the last 4 weeks.

Isn't a month four weeks long?

Actually, in a typical year, February is the only month that's four weeks (or 28 days) long. All others are either 30 or 31 days. On average, a month is 4.3 weeks, so that means the number of weeks and months of pregnancy don't match up exactly. And that's why some months in the chart are four weeks long and some are five weeks long.

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Is pregnancy nine or ten months?

Forty weeks is actually a little more than 9 months. For example, if your last period started on January 1, your due date would be October 8. So that's more like nine months and one week (or even longer if you go past your due date).

So, when will I have my baby?

Your provider counts 280 days (40 weeks) from the first day of your LMP to determine your due date. But keep in mind that's just an estimate. Only 5 percent of babies are born on their due date. You're just as likely to deliver any time during the two weeks before or after that day. Your baby is considered full term between 39 and 41 weeks.

Note: Not everyone ovulates exactly two weeks after their LMP, so your due date may be adjusted if an early ultrasound indicates that your baby is more or less developed than expected.

Follow your baby's amazing development

BabyCenter's editorial team is committed to providing the most helpful and trustworthy pregnancy and parenting information in the world. When creating and updating content, we rely on credible sources: respected health organizations, professional groups of doctors and other experts, and published studies in peer-reviewed journals. We believe you should always know the source of the information you're seeing. Learn more about our editorial and medical review policies.

ACOG. 2013. Definition of term pregnancy. American College of Obstetricians and Gynecologists. a new window [Accessed February 2021]

Jukic AM et al. 2013. Length of human pregnancy and contributors to its natural variation. Human Reproduction 28(10):2848-2855. a new window [Accessed February 2021]

Kate Marple
Kate Marple is a writer and editor who specializes in health, pregnancy, and parenting content. She's passionate about translating complicated medical information into helpful pregnancy and parenting advice that's easy to understand. She lives in San Francisco with her family.