How Many Weeks is a Pregnancy? All You Need to Know

Days, weeks, months, trimesters… all these date-related terms can get pretty confusing.

So, how many weeks is a pregnancy?

Let’s clear things up a bit.

The pregnancy countdown can be exciting, a little scary, and pretty darn confusing.

Days, weeks, trimesters, full-term… there are so many date-related terms.

You might be 13 weeks pregnant, but is that the first trimester or second?

How long is a pregnancy? How many weeks is a full term pregnancy? And how many weeks is a pregnancy, actually?

Don’t worry, mama - we’ll help you figure out these and other pregnancy timeline questions.

Whether you’re planning a pregnancy or you’ve just found out you’re pregnant, knowing your timelines can be helpful.

Let’s clear things up a bit.

In this article: 📝

How many weeks is a pregnancy?

There are 40 weeks in a pregnancy. Or 280 days.

Your due date — the last day of week 40 — is calculated from the first day of your last period, or last menstrual period (LMP).

Conception actually occurs about two weeks after the start of your period, during ovulation.

The LMP is used because trying to figure out when the sperm and egg met is not always as straightforward as it might seem.

Sometimes even if you know the date you had sex, it might not be the day you conceived.

(Sperm can survive in your uterus for up to five days!)

What is a due date?

An estimated due date (EDD) is the day your baby is expected to make their arrival.

Day 280 is considered your due date.

Medical professionals use this day to help plot your baby’s growth and development during pregnancy.

We have a quick and easy due date calculator to help you do the math.

It’s a good idea to remember that only 4% of babies arrive on their due date.

It is an estimated due date after all.

So it’s way more likely than not that you’ll have your baby either before or after your due date.

Try not to let that date stress you out.

How many trimesters in a pregnancy?

A pregnancy is made up of three trimesters.

The first trimester lasts from week 1 to week 13.

The second trimester starts at week 14 and ends at week 27.

And the third and final trimester of pregnancy starts from the first day of week 28 until week 40 (and beyond).

There are also some who refer to the fourth trimester, which technically isn't part of your pregnancy, but instead your transition from pregnancy to postpartum and motherhood.

How many weeks in a trimester?

There are 13 or 14 weeks per trimester.

Why not the same for each?

It’s simple: 40 doesn’t divide by three equally.

How many months is a pregnancy?

A pregnancy is 9 months.

You might be scratching your head at this point, wondering “but if a pregnancy is 40 weeks and you divide that by 4 weeks (because there are 4 weeks in a month), then you get 10 months. How does that work?”

Well, four weeks is 28 days, so February is actually the only month that is 4 weeks long.

So each month is actually 4.3 weeks long.

How many weeks is a full-term pregnancy?

Interestingly, what is considered a full-term pregnancy changes depending upon which country you live in.

The American College of Obstetricians and Gynecologists (ACOG) considers a pregnancy as full-term if it’s anything between the start of 39 weeks and the end of 40 weeks.

So from 1 week before your due date to 1 week after your EDD.

(That’s for a single baby. Full-term for twins is considered 38 weeks.)

• Preterm: Baby is born between 20 weeks and 37 weeks.
• Early term: Baby is born between 37 weeks and 38 weeks 6 days.
• Full term: Baby is born between 39 weeks and 40 weeks 6 days.
• Late term: Baby is born between 41 weeks to 41 weeks 6 days.
• Post term: Baby is born after 42 weeks.

The UK National Health Service (NHS), on the other hand, defines a full-term pregnancy at 37 weeks.

So, depending on where you live, different terms might be used!

So, how long is pregnancy?

280 days. 40 weeks. 9 months. 3 trimesters.

If you’re wondering what the next 40 weeks have in store for you, we’ve put together a useful week-by-week guide so you know what to expect.

And here’s a pregnancy checklist to help you through, month by month.

We’re excited to share the journey with you!