Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from the day you conceived. This means the clock starts about two weeks before fertilization actually happens. A full-term pregnancy is 40 weeks, or 280 days, measured from that starting point.
Why Counting Starts Before Conception
This is the part that confuses most people. The standard method adds an extra two weeks at the beginning of pregnancy when you weren’t actually pregnant yet. Ovulation and conception typically happen around day 14 of a 28-day cycle, so when your provider says you’re “4 weeks pregnant,” the embryo is closer to 2 weeks old.
This system exists because most people can reliably remember when their last period started, but very few know the exact day they ovulated or conceived. The gap between gestational age (counted from your LMP) and the actual age of the embryo stays constant throughout pregnancy. If you’re told you’re 12 weeks along, the embryo has been developing for roughly 10 weeks.
How to Calculate Your Week Right Now
Start with the first day of your last menstrual period. Count the number of days from that date to today, then divide by 7. The whole number is your week, and the remainder is extra days. So if it’s been 58 days since your LMP, you’re 8 weeks and 2 days pregnant (58 ÷ 7 = 8 remainder 2). Providers write this as “8+2” or “8w2d.”
Most pregnancy apps and online calculators do this math automatically. You enter your LMP date and they tell you your current week, your trimester, and your estimated due date.
Estimating Your Due Date
The classic formula for calculating a due date is called Naegele’s Rule, and it works in three steps. Take the first day of your last menstrual period, count back three calendar months, then add one year and seven days. If your LMP was March 10, 2025, you’d count back to December 10, then add a year and seven days to get December 17, 2025.
This formula assumes a 28-day cycle with ovulation on day 14. It’s a starting estimate, not a guarantee. Only about 5% of babies arrive on their exact due date.
What Happens With Irregular Cycles
If your cycles are longer than 35 days, shorter than 21 days, or unpredictable, the LMP method becomes less reliable. A longer cycle means you likely ovulated later than day 14, which would make the LMP-based count overestimate how far along you are. A shorter cycle could mean the opposite.
In these situations, ultrasound becomes the primary dating tool. An early scan, usually before 13 weeks, measures the length from your baby’s head to bottom. This measurement is compared against population data to estimate gestational age with high accuracy. If you don’t know when your last period was at all, or if your periods were very irregular, an early ultrasound is the most reliable way to date your pregnancy.
How Ultrasound Refines the Estimate
A first-trimester ultrasound (before 14 weeks) is the most accurate method for confirming gestational age. If the ultrasound date differs from your LMP-based date by more than 7 days, your provider will typically adjust your due date to match the ultrasound. That adjusted date then becomes the official one used for the rest of your pregnancy.
Accuracy decreases as pregnancy progresses. In the second trimester (14 to 22 weeks), ultrasound dating has a margin of error of 7 to 10 days instead of 7. Multiple measurements are taken at that stage, including the head, thighbone, and abdomen, but they provide a less precise estimate than the single head-to-bottom measurement used earlier on. Once a reliable first-trimester ultrasound has been done, your due date generally won’t be changed based on later scans.
Why Blood Tests Can’t Tell You Your Week
You might wonder whether the pregnancy hormone measured in blood tests can pinpoint how far along you are. It can’t, and the reason is simple: the normal range at any given week is enormous. At 6 weeks, for example, levels of that hormone can range from 1,080 to 56,500. Two people at exactly the same gestational age can have wildly different numbers. The trend matters more than any single value, which makes blood tests useful for monitoring a pregnancy’s health but useless for dating it.
Trimesters and Week Ranges
Once you know your current week, here’s how it maps to the three trimesters:
- First trimester: Week 1 through week 12. This is when the major organs and structures form.
- Second trimester: Week 13 through week 27. Growth accelerates and you’ll typically start feeling movement.
- Third trimester: Week 28 through week 40. The baby gains most of its weight during this stretch.
Pregnancies that reach 37 weeks are considered early term. Full term is 39 to 40 weeks. Deliveries at 41 weeks are late term, and 42 weeks or beyond is post-term.
Conception Date vs. LMP Date
If you know the exact date of conception, perhaps through IVF or tracking ovulation precisely, you can calculate your gestational age by adding two weeks to the embryo’s actual age. A pregnancy that resulted from conception on January 14 would be dated as though the LMP was January 1, putting you two weeks ahead of the embryo’s true developmental age. This keeps your dates consistent with the standard system your provider uses for milestones, growth charts, and scheduling.
IVF pregnancies follow the same logic. Providers calculate gestational age based on the embryo transfer date and the known age of the embryo at transfer, then convert that into the equivalent LMP date so everything lines up on the same 40-week timeline.