How can I calculate how many weeks pregnant I am?

You take a pregnancy test and see the two stripes. The results are positive: you’re pregnant. Now what? Whether you decide to continue with the pregnancy or not, a good first step is to calculate how many weeks you’ve been pregnant (this is called calculating your gestational duration, or gestational age). That way, you'll know your options.

Generally, if you go to see a doctor or nurse, they’ll give you an ultrasound and tell you how far along you are in your pregnancy. You can also calculate the gestational duration, even before you go to your first appointment. Here’s how to do the math.

How to calculate gestational duration

Gestational duration is counted from the first day of your last period. This can be confusing, because this method starts counting the days since your last period, or about three to four weeks BEFORE your actual pregnancy occurs. This is because your last period started a new cycle, in which you released an egg that was fertilized.

Although ovulation usually happens two weeks after your period, this isn’t guaranteed and can happen sooner or later, even if your cycle is regular. So since you can’t know for sure when you ovulated, the count begins from your last period, to keep things simple. Gestational duration is an approximation.

So let’s try this out. If today is January 18, and the first day of your last period was December 5, then we count the days between those two dates, which would be 44 days. This means you are approximately six weeks and two days pregnant. These calculations can be done by hand, using a calendar. But there are also plenty of gestational duration calculators online, making things even easier.

What if you don't know or don't remember the first day of your last period? No worries — a lot of people don’t remember the exact date, and this is normal. A nurse or doctor can figure out gestational duration with an ultrasound.