Pregnancy

How to Count Pregnancy Weeks

Team Peanut4 months ago4 min read

Your belly is expanding, your hormones are erupting, and your brain has decided that this is the perfect moment to get the heck out of dodge. And through all this, you are supposed to know how to count pregnancy weeks? It’s all a bit much, really.

How to Count Pregnancy Weeks

You know this pregnancy journey should be in the region of 9 months. But what is that in weeks? And when do you even start counting?

While your healthcare provider will help you keep on top of the timeline, it’s always good to have your own grasp on things. Knowing what to expect at each stage of the journey can help make the whole ride more comfortable. (Also, not a bad idea to form a mama community on Peanut. Nothing like going through it all together.)

So here’s the deal. We’re going to simplify the job of counting pregnancy weeks so that there’s one more thing off your to-do list.

How to count weeks of pregnancy

The first thing to know?

38 to 42 weeks is the average pregnancy span.

That being said, anomalies abound on either side of the spectrum. You can still have a perfectly healthy baby and a perfectly healthy mama if your pregnancy runs longer or shorter than average. (This baby born at 21 weeks is now a healthy toddler, and this mama was pregnant for a whole 12 months.) Every pregnancy story (and timeline) is unique.

So when do you actually start the count? Warning: the answer may not be as intuitive as you think.

Why is pregnancy counted from last period?

Want to hear something crazy? By the time you start counting, you may already be 4 weeks pregnant!

That’s because Day 1 of pregnancy is calculated from the 1st day of your last menstrual period, not from the date of conception. (That LMP acronym that you may have heard tossed around? Yes, that’s what it stands for: Last Menstrual Period.)

Why is 2 weeks pregnant actually 4 weeks?

We can’t count from the date of conception because it’s pretty hard to know for sure when conception took place. While we know that ovulation (usually happens about 14 days after the first day of your menstrual cycle), this is by no means an exact science.

So the medical profession decided that to make things easier, it would be better to start the count from the 1st day of your LMP.

That means that while you might be only 2 weeks away from conception, you may be 4 weeks away from the 1st day of your LMP—and as a result, “4 weeks pregnant.”

Once you have the LMP, you can start working out the estimated delivery date (EDD).

How to know how far along you are in pregnancy

To start, “9 months of pregnancy” is a bit of a misnomer. The 40 weeks of pregnancy don’t fit neatly into our calendar months, so there’s a little bit of fudging when counting pregnancy weeks to make it all fit. Some healthcare providers or other sources will divide the months up differently than we have here.

Here is a breakdown of your pregnancy into weeks, months, and trimesters.

  • First Trimester: Runs from Week 1 to Week 13. This feels like the shortest of the trimesters. Why? Because for the first bit of it, you might not even know you’re pregnant.

    Here’s your (rough) cheat sheet to the months in the first trimester:

    • Week 1 - 4: 1st month
    • Week 5 - 8: 2nd month
    • Week 9 - 13: 3rd month
  • Second Trimester: Runs from Week 14 to Week 27, and is often considered the easiest of the three.

    These are the weeks and months in the second trimester:

    • Week 14 - 17: 4th month
    • Week 18 - 22: 5th month
    • Week 23 - 27: 6th month
  • Third Trimester: Runs from Week 28 to the end. The last chunk puts you in full prep mode while carrying a small bowling ball inside of you.

    Here’s the timeline for your third trimester:

    • Week 28 - 31: 7th month
    • Week 32 - 35: 8th month
    • Week 36+ : 9th month

Along the way, you will likely have at least 2 ultrasounds—one at the beginning of the second trimester and one at the end. (You may have more than this, particularly if your healthcare provider needs to monitor things more closely.)

The first ultrasound is often called the “dating” ultrasound and can provide more accurate intel on how far along you are.

Okay, mama. We’re rooting for you. You’ve got this.

You might also be interested in:
Embryo vs Fetus: What’s the Difference?
What To Do When You Find Out You’re Pregnant