When you break the news about the pregnancy, the most common question you will hear will be: “How many weeks are you pregnant?” And, even though it may seem complicated to count how many weeks there are until the due date, it’s not!

Today we’re going to talk about the pregnancy math. We know that it can be hard to track down how many weeks there are until the due date. So, for those that are eager to master the pregnancy math, we’ve pieced together this article that’s going to provide you with relevant answers.

In order to figure out where you fall in, what week and trimester, it’s important to know what you should be counting. And, we’re going to shed some light on everything there is to know about how many weeks, trimesters and months there are in one pregnancy. After reading this, you’ll know for sure at what stage you are!

**The Number of Weeks in a Pregnancy**

Starting from the very first day after the last menstrual period, pregnancy lasts for 280 days or 40 weeks. However, we’d like to note that not every pregnancy will reach the 40th week. In fact, the vast majority of pregnancies ends at the 39th week.

On the other hand, there are some that last for 41 weeks. Both of these cases are considered to be full-term pregnancies. That means that a newborn that was delivered at 39 weeks isn’t an “early-born baby” — it’s exactly the same as the one born during the 41st week.

**The Number of Weeks in a Trimester**

A trimester is a term we use in order to indicate a period of time that lasts around 13 or 14 weeks. The reason why the week breakdown isn’t as exact as you might have thought is the fact that you should divide those 40 weeks by three.

**The Number of Trimesters in a Pregnancy**

A full pregnancy has three trimesters in it. The first trimester lasts from week one to week 13. Afterward, the mom-to-be steps into the second trimester that goes from week 14 until week 27. Finally, the third trimester lasts from week 28 until the day of delivery.

**Does Pregnancy Last 9 or 10 Months?**

As we’ve already mentioned, a pregnancy typically lasts for 40 weeks. This is counted as nine months. However, since a month is four weeks long, this would make the pregnancy last for ten months, right?

Well, not really. Every month, except February, has 30 or 31 days. That way, they are 4.3 weeks long, which means the pregnancy lasts for 9 months.

**Here’s How to Determine At What Point of the Pregnancy You Are **

This is something that will sound familiar — the doctor tells you that you’re 16 weeks pregnant. At the same time, they are telling you that you’re in week 17. Well, it’s no wonder moms get confused! However, the answer is that both of these counts are right! How?

Well, in both cases, the due date stays the same. The only thing that is different is the language people choose to use when doing the pregnancy math.

Here’s the thing — the number of weeks you’re into pregnancy differs from the number of weeks you’re pregnant. So, if you’re in week 17, you’re 16 weeks and a couple of days pregnant. Essentially, sixteen full weeks have passed; however, the 17th week hasn’t passed yet. That’s why it actually makes sense that some say you’re 16 weeks pregnant and in the 17th week of pregnancy.

Still confused? Don’t worry — we’ve got your back! Let’s put it like this; think about counting time as you would for a birthday.

For instance, when a person turns 1, they have lived through their first year and have initiated living their second year of life. Basically, turning one means you’re stepping into the second year. And then, in the same manner, when you celebrate your second birthday, you’re stepping into the third year of life.

Now it has to make more sense, right? You can count the weeks of the pregnancy the same way. So, you can be 16 weeks pregnant, but at the same time, you’re stepping into the 17th week of the pregnancy.

**The Proper Way to Calculate the Due Date**

In general, your obstetrician will be able to tell you how many weeks there are until the due date. There is a simple way to calculate that. So, in case you haven’t checked with your doctor when your due date should be, here’s how you can calculate it yourself.

Instead of starting to count the days from the day of conception, you should start counting from the last period you’ve had.

Assuming you know when the first day of your last period was, add those 40 weeks and voila — you’ll know your due date. Still, if you want to double-check and make sure you’re prepared for the date, during the next check-up with your doc, make sure you ask them about it.

**How to Calculate the Pregnancy the Right Way**

Many moms get confused when they learn that they shouldn’t start counting time from the day of conception. Instead, they should start counting from the last menstrual cycle.

We agree that it might be a bit confusing to count the pregnancy before the sperm has even met the egg. However, the last menstrual period date is a more reliable date to take into consideration.

Basically, it would be really hard to determine the exact moment of conception. So, even if you know the day you’ve ovulated and the day you’ve had sex with your partner, it would still be quite tricky to pinpoint the exact date.

The reason behind that is the fact that the time frame for conception is a slightly bigger window than what you might have thought. As a matter of fact, sperm can fertilize an egg even three to five days after arriving in the vagina. In addition to that, an egg can get fertilized for up to 24 hours after its release.

We’re sure it makes way more sense now, and it’s actually great news! This means that you’ve already gone through 4 weeks of being pregnant by the time you’ve missed your period.