How Many Weeks Pregnant Am I?

Here is a detail that surprises a lot of people: the pregnancy “clock” starts about two weeks before conception actually happens. That is because doctors and midwives do not count weeks pregnant from the day an egg is fertilized. They count from the first day of your last menstrual period, or LMP, and that single choice is the foundation for every due date math problem in this article. Once you know that starting point, the rest is simple, checkable arithmetic.

How “Weeks Pregnant” Is Counted From Your LMP

Gestational age is the standard clinical way to describe how far along a pregnancy is, and it is measured in weeks and days. The count starts on the first day of your last menstrual period, treated as day 0 of week 0.

From there, each block of 7 days adds one week. Day 7 after LMP is the start of week 1. Day 14 is the start of week 2, and so on, all the way to full term.

This convention feels odd at first, since you are not actually pregnant yet on day 0 of your LMP. Ovulation and conception happen later in that cycle. But because the exact day of ovulation is hard to pin down for most people, LMP gives doctors a clear, consistent starting line that works the same way for everyone.

• Week 0, day 0: first day of your last menstrual period.
• Weeks 1 through 40: counted forward from that date in blocks of 7 days.
• “How many weeks pregnant am I” simply asks how many of those 7 day blocks have passed since LMP.

The Simple Due Date Math

A full-term pregnancy is generally described as about 40 weeks from LMP. Multiply 40 weeks by 7 days, and you get 280 days. So the most direct formula is:

Estimated due date = LMP date + 280 days.

There is also a well known shortcut called Naegele’s rule, which lands on roughly the same date without counting 280 individual days. You take your LMP date, add 9 months, then add 7 more days.

Estimated due date = LMP date + 9 months + 7 days.

Both formulas are built on the same 40 week idea, so they usually land within a day or two of each other. The small gap happens because calendar months are not all the same length. Adding “9 months” moves you across a mix of 28, 30, and 31 day months, while counting 280 flat days does not care about month length at all. Neither shortcut is more official than the other. They are both estimates from the same LMP starting point, and a live Pregnancy Calculator can run either version for you instantly.

A Worked Example, Step By Step

Numbers are easier to trust when you can check them by hand. Say your LMP was March 1, 2026.

1. Count forward 280 days for the due date. March 1 plus 280 days lands on December 6, 2026.
2. Check it with the shortcut. March 1 plus 9 months is December 1, 2026. Add 7 days and you get December 8, 2026, just two days off from the 280 day count.
3. Find how many weeks pregnant that is on any given date. Count the days from March 1 to that date, then divide by 7.

For step 3, imagine today is September 14, 2026. From March 1 to September 14 is 197 days. Divide 197 by 7 and you get 28 remainder 1, which means 28 weeks and 1 day pregnant. That single number, “28 weeks and 1 day,” is the answer to “how many weeks pregnant am I” for this example, and it also means this pregnancy has just crossed into the third trimester.

You can rebuild this same math for any LMP date. The steps never change: count the days since LMP, divide by 7 for the week and day count, and add 280 days to LMP for the due date estimate.

The Three Trimesters, By The Week

Trimesters are simply labels for three roughly equal chunks of the 40 week timeline. They describe general stages, not health milestones, and the exact week that separates one trimester from the next can vary slightly between sources.

• First trimester: weeks 1 through 13.
• Second trimester: weeks 14 through 27.
• Third trimester: weeks 28 through 40.

Using the worked example above, 28 weeks and 1 day falls right at the start of the third trimester. The chart below shows how the three labels line up across the full 40 week span.

Weeks Pregnant vs Weeks Since Conception

This is the single biggest point of confusion in pregnancy dating. “Weeks pregnant” almost always means weeks since LMP, the number your doctor, your app, and your due date math all use. “Weeks since conception” is a different, smaller number.

Conception usually happens around ovulation, which is roughly 2 weeks after the first day of a typical menstrual cycle. So when someone says they are “10 weeks pregnant,” the embryo itself is usually closer to 8 weeks old. Both numbers are correct. They are just answering different questions.

Mixing the two up is an easy mistake, especially when reading about fetal development online, since some sources describe growth by conception age instead of gestational age. The LMP based gestational age is the one used for weeks pregnant and for due date calculations everywhere in this article.

Why Ultrasound Dating Can Adjust The Estimate

LMP based math assumes a regular, textbook 28 day cycle with ovulation on day 14. Real cycles vary a lot, so the LMP estimate is not always exact.

Early ultrasound, usually done in the first trimester, measures the size of the embryo or fetus directly. Because early growth is very consistent across pregnancies, that measurement is often a more accurate way to estimate gestational age than LMP alone, especially for irregular cycles.

When ultrasound dating and LMP dating disagree by more than a small margin, a doctor or midwife will often set the due date using the ultrasound measurement instead. This is a normal, routine adjustment, not a sign that anything is wrong. It simply reflects better information becoming available.

Why Your Due Date Is An Estimate, Not An Exact Day

It helps to think of the due date as the center of a range rather than a fixed appointment. Most pregnancies do not end on the exact estimated date, and a spread of a week or two on either side is common and expected.

The LMP math in this article gives you the same general estimate that standard dating conventions produce, and it is a useful way to understand where a due date number comes from. It is not a substitute for your own care.

• Use the math here to understand and double check the general idea behind a due date.
• Use ultrasound dating and your provider’s records for the most accurate estimate for your pregnancy.
• Bring any questions about your own dates, timeline, or care to an OB-GYN or midwife.

Frequently Asked Questions About How Many Weeks Pregnant Am I

Sources

This article is for general education only, not medical advice. Every pregnancy is different, and only a doctor or midwife using your own exam and ultrasound dating can give you an accurate assessment, so always follow their guidance over any calculator. Reviewed for accuracy by Dr. Abdullah Khalil, MBBS. Last updated September 14, 2026.