How Many Weeks Pregnant Am I? An Explanation Of Due Date Calculations

How many weeks pregnant am I is one of the most common questions you are going to ask yourself when you find out you're pregnant.

Has anyone ever noticed that pregnancy math isn't quite the same as normal math? It's funny because many women have trouble calculating how far along they are and get confused about due date, gestation, and when they actually conceived. If you frequently find yourself asking “how many weeks pregnant am I, again?”, you are not alone.

Pregnant women have trouble figuring out how many weeks or months pregnant they are because their due date doesn't quite match up with when they thought they got pregnant. Not to worry, we've all used a due date calculator a time or two.

I promise, your doctor didn't just throw a random date at you because it sounded good. There are specific calculations that are measured in order to give you your due date and let you know exactly how many weeks pregnant you are.

This post is going to give you the whole run down so that you always know the answer to your burning question, “How many weeks pregnant am I?”.

How many weeks pregnant am i?

There are a few ways to determine how many weeks or months pregnant you are. In other words, there are a few ways to determine when your due date is or how long you've actually been pregnant. Is it weird that all of those things sound like they mean the same thing but they actually mean slightly different things? Yikes! Let me explain.

Please note: I am not a doctor and I am not giving medical advice. Information on this website is not intended to diagnose, or treat any form of any disease. This article is for informational and entertainment purposes only. I am only telling you what worked for me. Please refer to my Disclaimer Page for more information.

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Determining when your due date is

The first thing that I know all women want to know when they find out they are pregnant is when their due date is. Am I right?

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Yeah, we all want to know exactly how many weeks or months we are, but THE DATE is the exciting part, isn't it?

Well, in order to figure out your due date, there are a couple of methods that you can go about it.

Due Date By Last Menstrual Period (LMP)

Any time that you go into your OBGYN's office for a pregnancy confirmation, they are going to make sure they get an accurate LMP to put in your chart.

The LMP is the date that they use to give you your approximate due date until you can get a more accurate measurement via ultrasound.

What date is the LMP, exactly?

Well, hopefully you've been keeping track of when your periods start and end. The LMP is the first day of your last menstural period. It's the day your period started last month.

Once you have that day in mind, you can easily calculate your estimated due date! There are two easy methods to pinpointing the day:

Add 280 days from your LMP

Subtract 3 months from your LMP and then add 7 days (and on year, obviously)

The second option seems more complicated but it's actually easier. Here is an example:

LMP was June 16, 2019
Subtract 3 months = March 16, 2019
Add 7 days (and on year) = March 23, 2020
Due date is March 23, 2020

Due Date Based on The Day You Conceived

For women that have irregular cycles, LMP might not be as accurate of a measurement for determining due date. Instead, if you know the day you conceived, you can calculate your due date based on that date instead.

The only way that you're going to really know when you conceived is if you have an IVF transfer date or you were diligently tracking ovulation and sex.

If you are certain of the day you conceived, then you can determine your due date.

All you need to do is add 266 days from you conception date to get your estimated due date.

Due Date Based on Ultrasound

Most of the time, if you get into your OBGYN's office within the first trimester, having an early scan to determine due date is the most accurate due date estimate.

If you have irregular cycles, don't remember the first day of your last period, and have no idea when you conceived, then having an early ultrasound is your best bet to get an accurate estimated due date.

Your doctor will usually use a transvaginal ultrasound to get a good picture of your baby and measure the crown to rump length. This basically just means that they are measuring your baby from top to bottom. That measurement will give a precise due date based on the size of your baby.

Healthy embryos are extremely predictable in their growth patterns in the first trimester. That means that using this method is pretty precise.

What your due date really means

Alright, so now you have your due date. So exciting!

But what does this date really mean?

Your due date isn't a deadline. Remember that. It's basically just a point in time that indicates an estimated date of delivery. In other words, a very-educated guess.

Truthfully, only 5% of women worldwide deliver on their calculated due date. Just because this is the date circled on your calendar, does not mean that this is going to be the date that your baby will arrive.

Your due date is simply the day that you turn 40 weeks pregnant.

How many weeks are in a pregnancy?

There are approximately 40 weeks (280 days) in a typical pregnancy. This countdown begins on the first day of your last menstrual period.

That being said, a TERM (or early term) pregnancy begins at 37 weeks. Once you hit this momentous landmark, you are no longer at risk for delivering pre-term (any time before the 37th week) and you can rest assured that spontaneous labor at this point is safe for your baby.

A FULL TERM pregnancy is anywhere between 39 and 41 weeks pregnant. If you deliver between 39 and 41 weeks pregnant, then you are right on time.

Between 41 and 42 weeks pregnant is considered LATE TERM. You might even hear the phrase “post dates” or “post term”.

The vast majority of women will deliver their babies within 10 days of their due date. That might mean 10 days before or 10 days after the expected date of delivery.

How many weeks are in a trimester?

Each trimester of pregnancy is about 3 months long. There are approximately 13 or 14 weeks in each trimester.

Your first trimester lasts approximately 13 weeks from the first day of your last menstrual period to the last day of your 13th week.

The second trimester begins on the first day of your 14th week and ends on the last day of the 27th week.

Lastly, your favorite trimester of all…the third trimester. The third trimester begins at 28 weeks pregnant and ends when you deliver your baby (somewhere around 40 weeks pregnant).

So is pregnancy actually 10 months long?

Well, yes and no.

Everyone knows that pregnancy is nine months long, right? So why is it that pregnancy lasts for 40 or even 41 weeks? That sounds like 10 months to me!

Wait a minute. Let's talk this out.

Most of us think of months as a four week time frame. If you have that mindset, then, yes, it looks like pregnancy actually lasts for 10 months.

But…

Reality is, most months on the calendar add up to be about 4 and a half weeks long. Not every month has 28 days (4 weeks), right? Most months out of the year have either 30 or 31 days. If you add it up, a perfect, 40 week pregnancy is approximately 9 months and one week long.

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

Determining how many weeks you are depends on where you're at in the process. Do you have a due date yet? Have you seen your provider? Do you know your LMP or ovulation date? All of those things will help you figure it out.

Determine how many weeks you are by:

Counting backwards from your due date– Your due date lands at 40 weeks and 0 days. So count backwards from there to find out how many weeks pregnant you are today.
Counting forwards from the date of your last appointment – At your appointment when you found out your due date, your provider likely told you how many weeks you were on that day. If you were 8 weeks and 1 day, count forwards until you figure out how many weeks you are today!
Use a pregnancy calculator– If you don't have a due date yet or haven't had your first appointment yet, use a pregnancy calculator website like this one to enter in your LMP or the day you ovulated to find out how many weeks you are today.
Use a pregnancy app – I love my Glow app. Glow has THE BEST pregnancy calendar so that you can look back, look forward, and see today so that you always know how many weeks pregnant you are and how many weeks pregnant you will be at any given time.
Keep a pregnancy journal – keeping a journal is always the best way to keep track of how many weeks pregnant you are. The Ultimate Pregnancy Journal and Planner has daily and weekly journal entries, trimester updates, AND a two year calendar for you to keep track of your dates.

How to keep track of how many weeks pregnant I am

So, you're going to have to get used to reporting your gestation in weeks and days. For example, you might find out that you are 8 weeks and 4 days pregnant at your first appointment. When you check into labor and delivery to be monitored or checked, they are going to ask how many weeks pregnant you are. Your answer will be in the same format, “I am 39 weeks and 2 days”.

Remember that you are still “36 weeks pregnant” when you are “36 weeks and 6 days”. Although in this case you are closer to 37 weeks than 36 weeks, when it comes to pregnancy, you will not be considered 37 weeks until midnight the next morning.

Think about it like how you think about age. When a baby turns one years old, they have just completed their first year. The very next day after their birthday, they are technically in their second year, right? But you don't say that they are two until 365 days from then. Whether you are 2 years and 3 days old or 2 years and 364 days old, you are still only 2 years old until your third birthday.

The same goes for pregnancy calculations. Even if you are in your “12th week of pregnancy”, you are only 11 weeks pregnant until you actually complete your twelfth week. Only at that point can you say that you are 12 weeks pregnant.

A helpful tip for keeping track of how many weeks you are

I always count how many weeks I am based off the day of the week. If I'm due on a Friday, that means that every Friday I advance to the next week. If I know that I was 8 weeks pregnant last Friday and today is Wednesday, I only have to count from there instead of counting all the way back or forward from my last appointment or due date.

I use this trick all of the time. Sometimes I forget how many weeks and days I am and I just have to recenter myself in the week. Is it Thursday? Ok, I know that tomorrow I'm going to go up a week. So if I was 21 weeks last Friday, then I am 21 weeks and 6 days today.

I hope that helps! Just keep track of your special day whether it's Friday or any other amazing day of the week, look forward to it each week to keep track of how many weeks pregnant you are!

How many weeks pregnant am I? And how many months is that?

Even I have a hard time remembering how many months pregnant I am because I am always so focused on the weeks.

It is more accurate to use weeks instead of months when referring to your pregnancy. Your pregnancy provider probably won't acknowledge months very much when determining how far along you are, how much longer you have left, or when to start intervening for the health of your pregnancy.

Weeks are where it's at.

Either way, here is a breakdown of how many months pregnant you are based on your weeks gestation.

First Trimester

1st Month: 1-4 weeks pregnant
2nd Month: 5-8 weeks pregnant
3rd Month: 9-13 weeks pregnant

Second Trimester

4th Month: 14-17 weeks pregnant
5th Month: 18-22 weeks pregnant
6th Month: 23-27 weeks pregnant

Third Trimester

7th Month: 28-31 weeks pregnant
8th Month: 32-35 weeks pregnant
9th Month: 36 – 40 weeks pregnant

Remember one thing about how many months pregnant you are. You are not technically one month pregnant until you reach the end of your first month. That makes sense, right? So when you are 4 weeks pregnant, now you can say that you are “one month pregnant”.

Wait, That doesn't make sense. I was on my period when I got pregnant?

Wait a second. How many weeks pregnant am I? Why do my calculations say that I got pregnant before I conceived?

I know, I know. Pregnancy calculations make it sound like you got pregnant the first day of your last menstrual period or two weeks before you ovulated. That doesn't make any sense, right?

Well, although we do calculate your pregnancy from that date, you are actually NOT PREGNANT for the first two weeks of your “pregnancy”.

MIND BLOWN.

Yeah, in a 40 week pregnancy, you conceived about 38 weeks ago. If you make it to your due date, your baby had been growing for about 38 weeks.

The crazy thing is that you've already knocked out 4 weeks of your 40 week pregnancy by the time you realize that you've missed your period. You know what else that means?

It means that the roughest trimester (in my opinion) is also the shortest! Out of 13 weeks, you really only know that you are pregnant for about 9 of them and symptoms don't typically show up until between weeks 6 and 8 anyway. Winning!

Why is pregnancy calculated from Your period instead of from ovulation?

So, if we aren't really pregnant for the first two weeks of our “pregnancy”, why do we even do it that way?

You're so right. It doesn't seem to make sense to start counting down the days of pregnancy before your egg has even left your ovary.

Unfortunately, most women don't know the exact date of conception. Using the first day of your last menstrual period is the most reliable way to calculate due date.

In order to make an attempt at changing the standards to calculate pregnancy based on ovulation, we would have to assume that everyone was tracking their ovulation. Ovulation isn't something that just happens obviously like a period where you actually have to take action when it happens (ie. wear a tampon or pad). Women have no idea that they are ovulating for most of their life and typically only start paying attention to it when they start actively trying to get pregnant.

This would pose a pretty huge problem since approximately 50% of all pregnancies are unplanned.

Let's just make it easy for everyone and keep the standards something easy for everyone to measure. We all probably notice when we get a period, right?

Can pregnancy by calculated from conception or implantation?

Even if you know exactly when you ovulated, pinpointing the moment of conception and implantation is almost impossible.

Sperm can live in a woman's body for 3-5 days as it waits to meet an egg. An egg doesn't have to be fertilized immediately either. An egg can actually be fertilized by sperm up to 24 hours after it has been released from the ovary.

So, even if you know the first day of your last menstrual period AND the day you ovulated, figuring out when the sperm fertilized the egg is not so easy. To take it a step further, to truly know when your body began to house life, you would have to know the time of implantation.

Not. Easy.

Related Read: First Trimester Bleeding: Why it Happens and What it Means

So how long have i actually been pregnant?

Ok, so now you've figured out your due date based on conventional standards and you and your doctor or midwife agree on your estimated date of delivery. But you still want to know exactly how long you've actually been pregnant. How long has this life been growing inside of you?

In order to find an answer that is satisfying for you, you need to know a few things first.

Lets go over a couple important definitions:

Ovulation: The time when your egg was released from your ovary.
Conception: When the sperm fertilizes the egg
Implantation: The attachment of the fertilized egg to the wall of the uterus

Now, what's really going down in your uterus when you get pregnant?

Well, first you have to ovulate. Women typically ovulate approximately two weeks after the first day of their period. After ovulation, the sperm meets the egg (conception) and then the fertilized egg has to travel down from the Fallopian tubes to the uterus. It takes about 6-12 days after conception for the fertilized egg to find it's spot in the uterus and actually implant. At this point, you are pregnant!

If you know the dates of all of these events, you are in the minority! Most women have no idea when they ovulated, when they conceived, or when their baby finally implanted. It's nearly impossible to know all of these things without help.

But hey, if you do then you know the most precise answer to How many weeks pregnant am I?

Remember that your providers are likely using conventional dating to determine your due date. Even though you have been growing life for 40 weeks, does not mean that you are 40 weeks pregnant by conventional standards. Typically, you have been growing your baby for about 2 weeks less than how many weeks pregnant you are determined by your doctor or midwife.

Does this mean that you should go off of the date that you've determined to be most accurate when making medical or health decisions for yourself or your baby? No. Finding out how long it's been since ovulation, conception, or implantation is just for your knowledge. Just so that you know how long you've really been pregnant. But when it comes to making medical decisions for yourself and your unborn baby, please DO trust your doctor or midwife when it comes to due date and how many weeks pregnant you are.

Figuring out how many weeks pregnant you are

Once you have figured out how many weeks pregnant you are, keeping track of how far along in your pregnancy you are will be a piece of cake. Before you know it you're going to be able to say “I am 21 weeks and 4 days” like it was the most natural thing in the world.

Trust me, by the time you are 40 weeks and 1 day, you are going to be counting the days, weeks, and months backwards and forwards trying to figure out why this baby is so darn comfortable in there. Trust me, I've been there.

Regardless of how you figured out your due date or how perfectly accurate it's going to be, you are going to have a baby! Congratulations! Now just try to sit back and be patient as best you can. There are a lot of things to get done from now until then so focus on those things instead of the long 9 months of waiting.

Don't know what you need to get done? Trying to figure out what the heck is on your To-Do List this week? Check out my Ultimate Pregnancy Journal and Planner to document your pregnancy, journal your highs and lows, and keep track of all of your appointments and To-Do lists all in one place.

How Many Weeks Pregnant Am I? An Explanation of Due Date Calculations