How far along am I? Pregnancy week calculator

How pregnancy weeks are counted

Doctors count pregnancy from the first day of your last period. That day is the start of week 1. Conception usually happens about two weeks later, around the time you ovulate. So in the first two weeks of a pregnancy, you aren't actually pregnant yet.

That's why the numbers can feel off. If you know the day you conceived, you're about two weeks further along than the time since then. A pregnancy lasts about 280 days, or 40 weeks, counted from your last period. That's about 266 days from conception.

The three trimesters are usually split like this:

First trimester: weeks 1 to 13
Second trimester: weeks 14 to 27
Third trimester: weeks 28 to 40

Why an ultrasound is more accurate

Counting from your last period assumes a regular 28-day cycle. Many people's cycles are longer, shorter or irregular, and it's easy to misremember a date. The American College of Obstetricians and Gynecologists (ACOG) notes that only about half of women recall the date of their last period accurately.

ACOG says an ultrasound measurement of the embryo or fetus in the first trimester (through 13 weeks and 6 days) is the most accurate way to set or confirm how far along a pregnancy is. If an early ultrasound and your period date don't match by much, a doctor may change your due date to match the ultrasound.

Knowing your real dates can take some of the panic out of things. Whatever you're thinking about, it helps to know how much time you have.

Get a free ultrasound

Many pregnancy centers offer a free, confidential pregnancy test and ultrasound with a licensed nurse or sonographer. You don't have to have made any decisions. Find free pregnancy help near you, or see centers in Michigan.

What's next

Fetal development week by week: what has formed at each stage.
Your options if you're pregnant and unsure: the support available, including help with money.

Sources

ACOG. Methods for estimating the due date, Committee Opinion No. 700 (2017).
Edwards KI, Itzhak P. Estimated date of delivery. StatPearls, National Library of Medicine (NIH), 2024.
MedlinePlus. Fetal development.
ACOG. How your fetus grows during pregnancy; Understanding and navigating viability.

Common questions

How do I figure out how many weeks pregnant I am?

Count from the first day of your last period. That day is the start of week 1, even though conception usually happens about two weeks later. The calculator on this page does the counting for you. An ultrasound in the first trimester is the most accurate way to know for sure.

Why is pregnancy counted from my last period and not from conception?

Most people don't know the day they conceived, but many remember when their last period started. So doctors count from that day. It means you're about two weeks further along than the time since conception.

How accurate is a due date based on my last period?

It's an estimate. It assumes a regular 28-day cycle, and ACOG notes that only about half of women remember the date of their last period accurately. ACOG says an ultrasound in the first trimester is the most accurate way to date a pregnancy.

Where can I get a free ultrasound?

Many pregnancy centers offer a free, confidential ultrasound. See free pregnancy help near you.

Last updated 2026-11-02. Medical facts on this page are drawn from the sources listed at the end and on our sources page. This is general information, not medical advice.