How can i calculate my weeks of pregnancy

Plenty of pregnant people are familiar with this scenario: You get a positive pregnancy test 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 you didn’t get pregnant four weeks ago while you were on your last period, 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 occurs. So health professionals use a universal standard for estimating how far along each pregnancy is by starting at your LMP, or 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, you’re already considered one week pregnant.

But keep in mind that this formula is based on a 28-day menstrual cycle, which is just the average cycle length. Some people have shorter cycles, some people have longer cycles, and that might affect your due date.

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 apps or calendars, don’t worry; most healthcare providers will perform an ultrasound when you’re around seven to eight weeks pregnant (called the “dating” or “viability” ultrasound) to get an early measurement of your baby. The size helps narrow down the amount of time the embryo has been developing. This scan is often taken as the most accurate measurement of your pregnancy weeks, so don’t be surprised if your provider has you jump forward or backward a week depending on the size of your baby.

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.

Your Pregnancy, Week by Week

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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 towards the end of the third trimester, every week kind of starts to blend together. Some pregnancy apps like Ovia or your Babylist registry have a pregnancy week counter on the homescreen, 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:

How Accurate is Your Due Date?

There’s certainly some level of accuracy to your EDD, but according to one British study, only about four percent of babies are born on their due date, and a 2013 Australian study found that about five percent of babies arrive on their exact 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 a 2017 report by the CDC, 26% of babies in the US were born two to three weeks before their due date, and over 57% 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.

This article was reviewed for factual accuracy in May 2022 by Dr. Christine Sterling, board-certified ob-gyn, member of the Babylist Health Advisory Board and founder of the Sterling Life, a resource for pregnancy and parenting.