You calculate how many weeks pregnant you are by counting forward from the first day of your last menstrual period (LMP), not from the day you actually conceived. This is the standard method used by doctors worldwide, and it means you’re technically “pregnant” for about two weeks before conception even happens. To find your current week, count the number of complete weeks between the first day of your last period and today.
The Standard LMP Method
The most common way to date a pregnancy is straightforward: take the first day of your last menstrual period and count forward. If your last period started on January 1 and today is March 12, you’d count the days between those two dates (70 days), then divide by 7. That puts you at 10 weeks pregnant.
This method assumes a 28-day menstrual cycle with ovulation happening on day 14. A full-term pregnancy is considered 280 days (40 weeks) from the first day of the LMP, and normal delivery can happen anywhere from 37 to 42 weeks. Because the count starts before ovulation and fertilization, gestational age is always about two weeks ahead of the actual age of the embryo. So when your provider says you’re 8 weeks pregnant, the embryo has really been developing for about 6 weeks.
How to Estimate Your Due Date
Once you know your LMP, you can estimate your due date using a formula called Naegele’s Rule. Johns Hopkins Medicine describes the three steps:
- Start with the first day of your last menstrual period.
- Count back 3 calendar months from that date.
- Add 1 year and 7 days.
For example, if your last period started on June 15, 2025, you’d count back three months to March 15, then add one year and seven days, giving you a due date of March 22, 2026. From your due date, you can work backward at any point in your pregnancy to figure out your current week. If your due date is 20 weeks away, you’re 20 weeks pregnant today.
If You Know Your Conception Date
When the exact day of conception is known, the math changes slightly. A pregnancy lasts 266 days (38 weeks) from conception, compared to 280 days (40 weeks) from the LMP. That two-week gap accounts for the time between your period starting and ovulation.
This distinction matters most for people who conceived through IVF or other assisted reproduction. In those cases, the due date is calculated by taking the embryo transfer date, subtracting the embryo’s age at transfer (typically 3 or 5 days), and then adding 266 days. If you had a day-5 blastocyst transferred on April 10, your estimated conception date would be April 5, and your due date would be 266 days later: December 27.
To convert a conception-based date into the standard gestational week count your provider uses, simply add two weeks. If it’s been 6 weeks since conception, you’re considered 8 weeks pregnant in gestational terms.
When Your Cycle Is Irregular
The LMP method works best when your cycles are predictable and close to 28 days. If your cycles are irregular, if you recently stopped hormonal birth control, or if you don’t remember when your last period started, the LMP date can be unreliable. You might ovulate significantly earlier or later than day 14, which throws off the count.
In these situations, an early ultrasound is the most accurate way to determine how far along you are. During the first trimester, an ultrasound measures the embryo’s size and estimates gestational age, often within a few days of accuracy. Research from Vanderbilt University has found that discrepancies between LMP-based dating and ultrasound-based dating are common and can result from ovulation dysfunction, irregular periods, or early pregnancy bleeding that gets mistaken for a period. If your ultrasound date differs significantly from your LMP calculation, your provider will typically use the ultrasound measurement to adjust your due date and week count.
Weeks and Trimesters at a Glance
Pregnancy is divided into three trimesters, each roughly three months long:
- First trimester: conception through 12 weeks
- Second trimester: 13 through 27 weeks
- Third trimester: 28 through 40 weeks
A detail that often causes confusion: when people say “I’m in my sixth week,” they typically mean they’ve completed five full weeks and are now in the days of week six. Pregnancy apps and medical records usually display this as “5 weeks and X days” or simply “5+3” for five weeks and three days. So “6 weeks pregnant” and “5 weeks, 4 days pregnant” are close but not identical, and the days notation is what your provider tracks at appointments.
Why the Numbers Sometimes Shift
It’s common for your estimated week count to change after your first ultrasound, and this doesn’t mean anything went wrong. The LMP calculation is a starting estimate. Ultrasound measurement in the first trimester is considered the most reliable way to confirm gestational age because embryos grow at a very consistent rate in early pregnancy. Later ultrasounds are less precise for dating because growth rates start to vary between individual babies.
If your provider adjusts your due date after an ultrasound, your week count shifts accordingly. A due date moved forward by a week means you’re one week further along than the LMP suggested. This can feel disorienting, but it simply reflects a more accurate picture of when ovulation and implantation actually occurred. After your due date is set (usually by the end of the first trimester), it typically stays fixed for the rest of your pregnancy.