You got a positive pregnancy test—yay! This realization is likely to lead to two very important questions: How many weeks pregnant am I? and When is my due date?
How Many Weeks Pregnant Am I?
Alright, mama—I’ve got a really easy answer for you. Simply put the first day of your last menstrual period (LMP) or the date you conceived (if you know it, that is) into our due date calculator. In seconds, this handy tool will return your expected due date, how many weeks pregnant you are, plus fun facts about baby’s development and mama’s changing body.
Why Does the Due Date Calculator Say I Was Pregnant Before I Even Conceived?
Wait, how many weeks pregnant am I? It makes sense to think that pregnancy starts at conception, since that’s the moment the sperm and egg unite to form your little bundle of joy.
However, conventional dating does not use conception as the starting point for pregnancy.
It is standard practice to date a pregnancy by the first day of your last menstrual period, because most women don’t know the exact date of conception.
Since most women do know the date of their last period, using the LMP is a lot easier.
Of course, this isn’t the most accurate way to date a pregnancy (more on that below). But it is the most popular way to figure out how due dates and gestation ages are calculated.
But what does this mean for those first two to three weeks of pregnancy? You aren’t technically pregnant at that time, so consider those days as freebies.
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Why Using Your LMP Isn’t the Most Accurate Method of Dating Pregnancy
Calculating the start of pregnancy by last menstrual period isn’t the most accurate method because there are many factors that affect when you ovulate. Picture this scenario:
- Mama 1: Starts period on Day 1, ovulates on day 14, and conceives on day 14. On day 28, she takes a test and it’s positive. She’s four weeks pregnant, but has only had life within her for two weeks.
- Mama 2: Starts period on Day 1, ovulates late on day 21, and conceives on day 22. On day 35, she takes a test, and it’s positive. She’s five weeks pregnant, but has had life within for only two weeks.
In the example above, both women started their period on the same day, and both women have had life within them for two weeks. But one woman is labeled four weeks pregnant and the other is labeled five weeks pregnant.
As you can see, the answer to “how many weeks pregnant am I” isn’t as easy as you’d think.
What’s the Most Accurate Way to Date a Pregnancy?
Going back to the hypothetical scenario above, it’s easy to see why the most accurate method for calculating how far along you are would be by implantation. Here’s the catch: It’s very hard to know when implantation occurs with certainty—even if you know when you ovulated.
After conception (when the sperm fertilizes the egg), the fertilized egg (which is called a blastocyst at this point) travels from the fallopian tubes into the uterus. Compared to the tiny blastocyst, the uterus is huge! There are many places for the blastocyst to implant, and it can implant in the upper area of your uterus or it may take the longer route to the lower area.
Because of the time it takes to travel to the implantation location, most experts agree that implantation occurs about nine days after conception, but it can range from as early as six days to as late as 12 days after conception. (source, source)
There is good news though! While there is no test to determine exactly what day implantation occurred, your body may give you hints that implantation has occurred:
What about your ovulation date?
After implantation, the most accurate method for dating pregnancy is to use your ovulation date. If you know when you ovulate, you have a much greater chance of accurately answering the question how many weeks pregnant am I?
But this isn’t a foolproof method either—you could be off by a few days, or you could make a miscalculation when trying to figure out when you will ovulate. (Try our ovulation calculator for a more accurate answer.)
How Ultrasound Can Help Date a Pregnancy
What if you don’t know the date of your last menstrual period? Or maybe your cycle is so irregular that you can’t even begin to guess when you may have ovulated? Don’t worry! A dating ultrasound can help; in fact, doctors commonly use a dating ultrasound as a tool to verify the LMP method.
You might receive a dating ultrasound at your first prenatal appointment or slightly later into your pregnancy.
Why is the ultrasound method so useful in calculating how many weeks pregnant you are?
Healthy embryos grow at a very predictable rate in the first trimester, so the crown to rump measurement can provide a more accurate estimate as to how many weeks pregnant you are. Crown to rump refers to the distance between the head (crown) to the end of baby’s bottom (rump). When dating ultrasounds are done before the 13th week of pregnancy, their accuracy is +/- five to seven days. (source) In the second trimester that jumps up to +/- seven to 10 days. (source)
ultrasound method – How Many Weeks Pregnant Am I Here’s the Most Accurate Answer
The above image depicts an early ultrasound used for dating purposes. (Image source)
If you get an official due date from your dating ultrasound, you can use our reverse due date calculator to see when you likely conceived, then you’ll have a much better answer for “how many weeks pregnant am I.”
What If Your LMP Due Date And Ultrasound Due Dates Aren’t The Same?
What if your due date (based on LMP) is different than the due date according to the ultrasound? Or what if you have four different due dates?! (Yes, it’s possible to have four different due dates: LMP, implantation (if you felt cramps), ovulation, and ultrasound.)
In these instances, your OB or midwife may adjust your due according to the ultrasound, since a dating scan is considered more accurate. (Check out this post to find out how accurate your due date is.)
Note: Many healthcare providers may not adjust the due date unless you’re more than a week off, but this can have implications at the end of pregnancy in terms of induction. If you feel like your due date needs to be adjusted, ask for a dating ultrasound or follow up with your doctor to adjust your due date. If your due date is not adjusted, you may hear talk about inductions when you’re one week “late,” even if that could actually be your true due date.
I’m 7 Weeks 1 Day Pregnant, So I’m In My 8th Week Of Pregnancy, Right?
Wrong. You are seven weeks pregnant.
Many moms assume that being 7 weeks and 1 day pregnant means they’re technically in their eighth week of pregnancy. And, while there is definitely logic behind that, it’s just not the way the medical community answers the question of “how many weeks pregnant am I.”
Think of it in terms of your age.
If someone asks you how old you are, you say you’re 30, whether you’re 30 and 3 days or 30 and 364 days. Even the night before your 31st birthday, you still say you’re 30, although you’ve almost completed your 31st year.
So how many weeks pregnant are you? Most mamas answer that by saying how many weeks and days they’ve completed:
How About You?
Which dating method did you use? Was it accurate? Did your healthcare provider end up changing your due date? Share your story in the comments below.