How to Count Your Pregnancy Weeks
Who would have thought counting would be this complicated?
By Amylia Ryan
Medically Reviewed by Christine Sterling M.D.
Fact Checked by Shannon Vestal Robson
In This Article
Plenty of pregnant people are familiar with this scenario: You get a positive result from one of the most accurate pregnancy tests a couple days after your period is late—presumably about two weeks after you’ve ovulated and conceived if you have a 28-day cycle. So you’re two weeks pregnant, right? You go see your healthcare provider to confirm your pregnancy, and they tell you you’re four weeks pregnant.
But it couldn't have happened four weeks ago, right? The chances of getting pregnant while you're on your period are slim to none, so what’s with the extra two weeks? You’d think counting wouldn’t be this complicated.
Confused? You’re not alone. Here’s the thing about baby making: no one can pinpoint exactly when conception and implantation happen. So health professionals use a universal standard for estimating how far along each pregnancy is by starting at your LMP (the first day of your last menstrual period).
How to Count Pregnancy Weeks
The basic formula is LMP + 280 days = EDD (estimated due date), and pregnancy week counting starts on the first day of your last period, starting with Week 0 (not 1!) and counting all the way up to Week 40 (your due date) and as high as Week 42. Here's another way to think about it: one week after the first day of your last period, your provider considers you to be one week pregnant.
Why all the math? "It's done this way because due dates and [human] pregnancy began thousands of years before the invention of sonography," says Dr. Joel Bernstien, an obgyn with Kamm McKenzie OBGYN in North Carolina. (We'll get to sonograms and your first ultrasound—and why they're more accurate—in a moment.)
"Due dates are given based on a 28-day cycle length," Dr. Bernstien says. "The assumption is a conception around cycle day 14. For example, if a couple is getting IVF and a five-day-old embryo is placed in the womb, this is equivalent to being on cycle day 19 by a due date calculation."
But keep in mind that 28 days is just the average cycle length. Some people have shorter cycles, some people have longer cycles, and that might affect your due date.
Math not your strong suit? Plug in your last known period into our due date calculator and we can help you figure it out.
How the first ultrasound can help you count your pregnancy weeks
What if you don’t remember exactly when the first day of your last period was? If you’re not in the habit of tracking your cycle with fertility apps or calendars, don’t worry; some healthcare providers will perform an ultrasound when you’re around eight weeks pregnant (called the “dating” or “viability” ultrasound—you can request one if your provider doesn't automatically do it) to get an early measurement of your baby.
Once your doctor has that measurement, they may use that instead of your LMP to determine how far along you are. "Early sonograms are more accurate than LMP dating because many women have irregular cycles or non-28 day cycles," Dr. Bernstien says.
The size measured in that first ultrasound helps narrow down the amount of time the embryo has been developing. "The early sonograms (let’s say <10 weeks), if done correctly, are super accurate as all babies grow essentially the same amount every day," Dr. Bernstien says. So don’t be surprised if your provider has you jump forward or backward a week or so depending on the size of your baby.
But even after the ultrasound results, your due date may stay the same. "If an early sonogram is close enough to dates by an LMP, the EDD is kept by the LMP," Dr. Bernstien says.
How to Count Pregnancy Weeks with IVF
If you conceived via in vitro fertilization, your pregnancy week calculation will look slightly different.
Using freshly harvested eggs: Week 2 begins on the day your eggs were retrieved.
Using frozen embryos: Week 2 begins on the day of the embryo transfer minus how many days old the embryos are. For example, if a 3-day-old embryo is transferred/implanted on February 12th, you’re considered two weeks pregnant on February 9th, and you can continue counting weeks from there. If a 5-day-old embryo is transferred on July 20th, Week 2 of your pregnancy started on July 15th.
Pregnancy Week Counter
Having trouble keeping track? We get it. With everything else you have going on, you may not have the brain space for constantly remembering your current pregnancy week—especially toward the end of the third trimester, every week kind of starts to blend together. Thankfully, some pregnancy apps like Expectful or your Babylist registry have a pregnancy week counter on the home screen, showing how many weeks pregnant you are or how many days you have left.
With some apps as well as at your prenatal checkups, you might notice some extra letters and numbers alongside your pregnancy week—something like 12w4 or 20w2. That’s your pregnancy week plus how many days into that week you are. So 12w4 is four days into the 12th week of pregnancy.
Pregnancy Trimesters
Trying to figure out what trimester you’re in? There’s no math involved for this, but it’s still not as intuitive as you’d think. Oddly enough, distinguishing pregnancy trimesters isn’t as simple as 40 weeks divided into three equal portions. Instead, they’re divided as follows:
First trimester: Week 1 through Week 12
Second trimester: Week 13 through Week 27
Third trimester: Week 28 through birth
How Accurate is Your Due Date?
There’s certainly some level of accuracy to your EDD, but they're not meant to be guarantees. "Due dates are by definition an estimation," Dr. Bernstien says. "Only five percent of women deliver on their actual due date."
So rather than basing baby’s arrival around a single day, it might be more accurate to consider it your due week, or even your due month. According to birth data collected by the CDC, in 2022 29 percent of babies in the US were born two to three weeks before their due date, and 55 percent were born either the week of or the week before their EDD, so there’s quite a bit of wiggle room as far as when your baby might arrive.
P.S. Here are some helpful (and fun) tips for each week of pregnancy.
