Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the day you actually conceived. That starting point means you’re already considered “2 weeks pregnant” around the time ovulation and conception typically happen. A full-term pregnancy lasts 280 days, or 40 weeks, counted from that LMP date.
The Basic Calculation
To figure out how many weeks pregnant you are right now, count the number of days from the first day of your last period to today, then divide by 7. If your last period started on March 1 and today is May 10, that’s 70 days, which puts you at exactly 10 weeks.
Providers track pregnancy in a “weeks and days” format. So if you’re 70 days from your LMP, you’re 10 weeks and 0 days. At 73 days, you’d be 10 weeks and 3 days. This level of precision matters because certain screenings and milestones are tied to specific windows measured in weeks and days.
How to Estimate Your Due Date
The standard method is called Naegele’s Rule, and it works 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.
For example, if your last period started on June 15, 2025, count back 3 months to March 15, then add 1 year and 7 days. Your estimated due date would be March 22, 2026. This formula assumes a regular 28-day menstrual cycle with ovulation happening around day 14. Once you have your due date, you can work backward from it to confirm how many weeks along you are at any point.
Why the Count Starts Before Conception
This is the part that confuses most people. The pregnancy clock starts about two weeks before you actually conceived, because it’s anchored to your last period rather than fertilization. Your baby’s actual developmental age (sometimes called fetal age) is roughly 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.
The reason everyone uses this system is simple: most people know when their last period started, but very few know the exact day they ovulated or conceived. The LMP gives a consistent, easy-to-identify starting point.
When Your Cycle Isn’t 28 Days
Naegele’s Rule assumes ovulation on day 14 of a 28-day cycle. If your cycles are longer or shorter, or irregular, the LMP calculation can be off. Someone with a 35-day cycle likely ovulated around day 21 instead of day 14, which means conception happened about a week later than the standard formula assumes. In that case, the LMP-based count would overestimate how far along you are by roughly a week.
This is one reason early ultrasounds are so valuable. In the first trimester, an ultrasound measures the embryo’s length (from the top of the head to the bottom of the spine) and can estimate gestational age with strong accuracy. If the ultrasound date and your LMP-based date don’t line up, your provider will typically adjust your due date and your week count accordingly. The ultrasound-based estimate is generally considered more reliable than the LMP calculation, especially for people with irregular cycles or uncertain period dates.
How Weeks Map to Trimesters
Once you know your week count, here’s how it fits into the three trimesters:
- First trimester: From the first day of your LMP through 13 weeks and 6 days. This is when fertilization happens and major organs begin forming.
- Second trimester: 14 weeks and 0 days through 27 weeks and 6 days. A period of rapid growth and development.
- Third trimester: 28 weeks and 0 days through 40 weeks and 6 days. The baby gains weight and organs mature in preparation for birth.
Knowing which trimester you’re in helps you track what screenings and milestones to expect. The anatomy scan, for instance, is typically scheduled between 18 and 22 weeks. Glucose screening usually happens between 24 and 28 weeks.
Calculating Weeks After IVF
If you conceived through IVF, the calculation works a bit differently because you know the exact date of embryo transfer. The pregnancy is dated from the transfer date rather than from a last menstrual period, and because there’s no two-week gap between a period and ovulation to account for, the total timeline is about two weeks shorter than a natural conception calendar. Your fertility clinic will provide a specific due date based on the transfer, and your week count flows from there.
A Quick Reference Method
If you don’t want to do math every time, pick your LMP date and note which day of the week it fell on. Each time that same weekday rolls around, you’ve completed another week. If your last period started on a Wednesday, then every Wednesday you tick over to the next week of pregnancy. Wednesday to Wednesday, 40 times, gets you to your due date.
Keep in mind that only about 5% of babies arrive on their exact due date. The 40-week estimate is a midpoint, not a deadline. Full-term birth anywhere from 39 weeks through 40 weeks and 6 days is considered perfectly normal timing.