Pregnancy is counted from the first day of your last menstrual period (LMP), not from the day you actually conceived. That means your “week count” starts about two weeks before fertilization likely happened. To find how far along you are right now, count the number of weeks and days from the first day of your last period to today’s date.
The LMP Method Step by Step
Pull up a calendar (your phone’s works fine) and find the date your last period started. Count forward from that date to today in complete weeks and leftover days. If your last period started on March 1 and today is May 10, that’s 10 weeks and 0 days, written in medical shorthand as “10+0.” You would be in your 11th week of pregnancy but referred to as “10 weeks pregnant,” the same way a baby who has lived 11 months is still “0 years old.”
This counting method assumes a 28-day menstrual cycle with ovulation on day 14. If your cycle is consistently longer or shorter, the estimate can be off by several days or more. A 35-day cycle, for example, means you likely ovulated around day 21 instead of day 14, so the LMP date would make you seem about a week further along than you really are. An early ultrasound is the most reliable way to correct for this.
Gestational Age vs. Fetal Age
The number you get from the LMP method is called gestational age. It includes roughly two weeks before conception, so your baby’s actual developmental age (fetal age) is about two weeks less than your gestational age. When your provider says you’re “8 weeks pregnant,” the embryo has been developing for closer to 6 weeks. Nearly all pregnancy milestones, test timings, and due dates use gestational age, so this is the number to track.
How to Estimate Your Due Date
The standard formula, known as Naegele’s Rule, gives you a due date in three steps:
- Step 1: Find the first day of your last menstrual period.
- Step 2: Count back 3 calendar months from that date.
- Step 3: Add 1 year and 7 days.
If your last period started on June 15, 2025, you’d count back to March 15, then add a year and 7 days to get March 22, 2026. This formula assumes a 28-day cycle and a full-term pregnancy of 280 days (40 weeks). Once you have your due date, you can count backward from it to figure out exactly how many weeks remain at any point.
When Ultrasound Changes the Dates
A first-trimester ultrasound is the most accurate way to date a pregnancy. Before 14 weeks, the measurement of the embryo from head to rump is accurate to within 5 to 7 days. Before 9 weeks, it’s even tighter: if the ultrasound disagrees with your LMP-based date by more than 5 days, your provider will typically change the due date to match the ultrasound.
In the second trimester (14 to 22 weeks), dating is based on a combination of measurements including the baby’s head, abdomen, and thigh bone. Accuracy drops to plus or minus 7 to 10 days during this window. After 22 weeks, ultrasound dating becomes less reliable still, because babies start growing at more individual rates. This is why that early scan matters so much for pinning down your timeline.
Dating an IVF Pregnancy
If you conceived through IVF or another assisted method, the math is more precise because you know exactly when the embryo was transferred. For a Day 5 (blastocyst) transfer, subtract 5 days from the transfer date to get the equivalent conception date, then add 266 days for the due date. For a Day 3 transfer, subtract 3 days instead. From there, you count weeks the same way as anyone else. Your gestational age will be the conception-equivalent date plus 2 weeks, since the standard system still includes those “pre-conception” weeks in the count.
Converting Weeks to Months and Trimesters
People will ask “how many months are you?” and the conversion is less straightforward than it sounds. Calendar months vary from 28 to 31 days, but pregnancy months are counted as exactly 4 weeks each. Here’s how it breaks down:
- Weeks 1 through 4: Month 1 (first trimester)
- Weeks 5 through 8: Month 2 (first trimester)
- Weeks 9 through 12: Month 3 (first trimester)
- Weeks 13 through 16: Month 4 (first/second trimester transition)
- Weeks 17 through 20: Month 5 (second trimester)
- Weeks 21 through 24: Month 6 (second trimester)
- Weeks 25 through 28: Month 7 (second/third trimester transition)
- Weeks 29 through 32: Month 8 (third trimester)
- Weeks 33 through 36: Month 9 (third trimester)
- Weeks 37 through 40: Month 10 (third trimester)
Yes, pregnancy is technically 10 “pregnancy months” long, even though we say 9 months colloquially. The first trimester runs through 13 weeks, the second trimester covers weeks 14 through 27, and the third trimester spans weeks 28 through 40.
What “Full Term” Actually Means
Not all weeks at the end of pregnancy are equal. The current medical definitions split the final stretch into distinct categories:
- Early term: 37 weeks through 38 weeks and 6 days
- Full term: 39 weeks through 40 weeks and 6 days
- Late term: 41 weeks through 41 weeks and 6 days
- Post-term: 42 weeks and beyond
This distinction matters because babies born at 39 weeks and later have better outcomes on average than those born at 37 or 38 weeks, even though all of these fall within what people casually call “full term.” Knowing your accurate gestational age helps you and your provider make informed decisions if questions come up about timing of delivery.
Why Blood Tests Don’t Pin Down Your Week
You might wonder whether the pregnancy hormone hCG, measured in early blood tests, can tell you how far along you are. In practice, the ranges overlap too much to be useful for dating. At 6 weeks, for example, normal hCG levels fall anywhere from 200 to 32,000. At 7 weeks, the range is 3,000 to 160,000. Two women at exactly the same gestational age can have wildly different hCG levels and both be perfectly normal. hCG blood tests are helpful for confirming that a pregnancy is progressing, but they can’t replace a calendar count or an ultrasound measurement for figuring out your week.
If You Don’t Know Your Last Period Date
Irregular cycles, breakthrough bleeding, or simply not tracking your period can all make the LMP method unreliable. In these cases, an early ultrasound becomes the primary dating tool. The earlier in pregnancy the scan is done, the more accurate it will be. If you’re unsure of your dates and haven’t had an ultrasound yet, scheduling one sooner rather than later gives you the most precise timeline to work with for the rest of your pregnancy.