Curious exactly which week you’re in — and what’s actually happening in there? Pop in the first day of your last period below, and we’ll map out all 40 weeks for you, trimester by trimester.
Weekly Pregnancy Calculator
See a week-by-week timeline of your pregnancy.
Most caregivers count pregnancy from the first day of your last menstrual period (LMP), not the date of conception. Enter that date and we'll lay out weeks 1–40.
Weeks are estimates based on a standard 40-week pregnancy counted from LMP. Your care team's dating may differ slightly, especially after an early ultrasound.
Counting pregnancy in weeks feels oddly precise for something that’s really an estimate, but weeks are still the most useful unit doctors have. Here’s what your week number actually means and why it can look different depending on where you check it.
Why Weeks Don’t Line Up Neatly With Months
Forty weeks doesn’t divide evenly into nine months, which is where a lot of the confusion comes from. Nine months is closer to 39 weeks, and pregnancy is usually described as 40 weeks from your last period, which is really about 38 weeks from conception. None of these numbers are wrong. They’re just measuring from different starting points, which is why “I’m five months along” and “I’m 22 weeks” can both be true at once.
Most of the weeks on this calculator happen before most people even know they’re pregnant, which trips people up every time. Doctors count from the first day of your last period, not from conception, so by the time you get a positive test, you’re often already three or four weeks “in.” If you want to work backward from when you likely actually conceived, that’s roughly two weeks later than your LMP date — which is exactly why the calculator above has that 14-day note built in.
Once you’ve got your weeks mapped out, it helps to zoom out to trimesters, since that’s really how your body (and your care team) thinks about the pregnancy: what changes trimester by trimester is a bigger shift than what changes week to week. Early on, a lot of what you’re feeling is hormonal and invisible from the outside; later, it’s obvious to everyone. Which symptoms show up and when varies a lot from person to person, so don’t panic if your week 9 doesn’t look like your friend’s week 9.
The week number is also a handy anchor for the other stuff you’ll want to keep tabs on what you’re eating, when your next appointment should land, how much weight gain is typical at this point. We’ve got dedicated tools for a couple of those: a prenatal appointment timeline so you know what’s coming up at the doctor’s office, and a weight gain chart if you want a sense of what’s typical week by week.
One thing worth remembering: this calculator gives you an estimate, not a diagnosis. An early ultrasound is usually more accurate than LMP-based math, especially if your cycles run irregular, so if your care team’s dates and this calculator’s dates don’t quite match, trust theirs.