The Standard Method: Counting From Your Last Menstrual Period

Pregnancy is counted from the first day of your last menstrual period, not from the day you conceived. This means you are considered about two weeks pregnant before conception actually happens. Your doctor will use this date—called the last menstrual period (LMP)—as the starting point for all pregnancy calculations.

To find your LMP, look back at your calendar and locate the first day of your last period before you became pregnant. Write down that date. From that date forward, you count every seven days as one week of pregnancy. This method is standard across hospitals, clinics, and ultrasound technicians, so your week count will be consistent no matter where you receive care.

The reason doctors use LMP instead of conception date is straightforward: most people do not know the exact day they conceived, but they do know when their period started. Using a consistent starting point means every pregnancy measurement—from due date to screening timelines—lines up the same way across all providers.

Key Takeaways

  • Pregnancy weeks are counted from the first day of your last menstrual period, which puts you at about two weeks pregnant on the day you actually conceive.
  • To count weeks yourself, count every seven days forward from your LMP date as one week of pregnancy.
  • An ultrasound in the first trimester can adjust your week count if your cycle is longer or shorter than the standard 28 days.
  • Your due date is calculated as 40 weeks from your LMP, though most babies arrive between weeks 38 and 42.
  • Pregnancy is divided into three trimesters of roughly 13 weeks each, and each trimester has different screening tests and milestones.

Counting Weeks Manually With a Calendar

Start with your LMP date and count forward seven days at a time. If your last period started on January 10, then January 17 marks the end of week 1, January 24 marks the end of week 2, and so on. You can do this on a paper calendar or a digital one—just mark every seventh day and number them.

After four weeks, you reach the end of the first month of pregnancy (though only about two weeks have passed since conception). After 13 weeks, you reach the end of the first trimester. After 26 weeks, you are halfway through pregnancy. After 40 weeks, you reach your due date.

If you want to know what week you are in right now, count the number of complete weeks that have passed since your LMP. If today is 15 days after your LMP, you are in week 2 (because one complete week has passed, and you are partway through the second). If today is 22 days after your LMP, you are in week 3.

Using an Online Calculator or Your Doctor's Measurement

You can enter your LMP date into a pregnancy calculator online, and it will show you your current week, your due date, and what trimester you are in. These calculators assume a standard 28-day cycle and a 40-week pregnancy, so they give you a baseline number that works for most people.

Your doctor or midwife will measure your pregnancy more precisely using an ultrasound, especially in the first trimester. An ultrasound can see the size of the embryo or fetus and compare it to standard growth charts. If your cycle is longer than 28 days (say, 32 days), your actual conception happened later than the LMP math suggests, and the ultrasound will show that. If your cycle is shorter, the opposite is true. Your provider will adjust your week count and due date based on what the ultrasound shows.

After the first trimester, ultrasounds become less precise at dating pregnancy because all fetuses are roughly the same size at that stage. So your LMP date (possibly adjusted by an early ultrasound) remains your official week count for the rest of pregnancy.

Understanding Trimesters and Key Screening Dates

Pregnancy is divided into three trimesters, each roughly 13 weeks long. The first trimester runs from week 1 to week 13, the second from week 14 to week 27, and the third from week 28 to week 40 (or beyond). Knowing which trimester you are in helps you understand what screening tests and physical changes to expect.

Certain screening tests happen at specific weeks. The first-trimester screening (blood work and ultrasound) typically happens between weeks 11 and 14. The anatomy ultrasound, which checks the baby's organs and structure, usually happens around week 20. The glucose screening for gestational diabetes often happens around week 24 to 28. Knowing your week count tells you when these appointments should be scheduled.

Your due date is set at 40 weeks from your LMP. However, pregnancies that deliver between weeks 38 and 42 are considered full-term and low-risk. If you have not delivered by week 42, your doctor will discuss induction or other next steps with you.

What Happens If You Do Not Know Your Last Menstrual Period

If you cannot remember your LMP date, your doctor will use an ultrasound to estimate how far along you are. An ultrasound in the first trimester can pinpoint your week count within a few days of accuracy. An ultrasound in the second trimester is accurate within about two weeks. An ultrasound in the third trimester is accurate within three to four weeks.

Once your doctor establishes a week count from an ultrasound, that becomes your official dating for the rest of pregnancy. You will not change it again even if later ultrasounds show slightly different measurements, because the earliest ultrasound is the most reliable.

If you find out you are pregnant very late (for example, in the third trimester), your doctor will do an ultrasound and estimate your due date based on fetal size. This estimate will be less precise than one based on LMP, but it is the best information available at that point.

Counting Weeks When You Know Your Conception Date

If you know the exact day you conceived (for example, if you used ovulation tracking or had fertility treatment), you can add 14 days to that date to find your LMP equivalent. Conception typically happens about 14 days after the first day of your last period, so working backward gives you the LMP date your doctor would use.

However, do not be surprised if your doctor's LMP-based week count differs slightly from what you calculate from your conception date. Ovulation and conception timing can vary, and your doctor will confirm the week count with an early ultrasound. Trust the ultrasound measurement over your own calculation, because it is based on actual fetal development rather than assumptions about your cycle.

Common Mistakes When Counting Pregnancy Weeks

The most common mistake is counting from conception instead of from your LMP. This makes you think you are about two weeks further along than you actually are by medical standards. If you conceive on January 24 but your last period was January 10, your LMP date is January 10, and that is the date your doctor uses—not January 24.

Another mistake is miscounting the days. Make sure you are counting complete weeks (seven full days) and not rounding. If your LMP was January 10 and today is January 20, you have completed one week and are in your second week—not two weeks yet.

Some people also assume their due date is exact. Your due date is the 40-week mark, but it is a target, not a may provide. Most babies arrive within two weeks before or after the due date. If you go past your due date, your doctor will monitor you closely and discuss next steps, but this is not unusual.

Frequently Asked Questions

How do I know if my week count is off?

An early ultrasound (before 14 weeks) is the most reliable check. If the ultrasound shows your baby is smaller or larger than expected for your LMP date, your doctor will adjust your week count. After 14 weeks, ultrasounds are less precise, so your original LMP-based count usually stays the same even if measurements suggest a slight difference.

Can I be further along than I think?

Yes, if your menstrual cycle is shorter than 28 days. A 24-day cycle means you ovulate earlier, so conception happens sooner after your LMP. An early ultrasound will catch this and adjust your due date earlier. The opposite is true for longer cycles—you may be less far along than the LMP math suggests.

What if I had irregular periods before pregnancy?

Irregular periods make LMP dating less reliable, so your doctor will rely more heavily on an early ultrasound to establish your week count. An ultrasound in the first trimester is still accurate within a few days, so you will have a solid dating even without a predictable cycle history.

Does my due date change after the first ultrasound?

It may shift by a few days if the ultrasound shows your baby is measuring ahead or behind your LMP date. After the first trimester, your due date usually stays the same even if later ultrasounds show different measurements, because early ultrasounds are more accurate for dating.

What week am I in if I am 20 days past my last period?

You are in week 3. One complete week is seven days, so at 14 days you finish week 2. At 21 days you finish week 3. At 20 days, you are partway through week 3 (specifically, six days into it).