How to Know How Many Weeks Pregnant | From Your Last Period

Gestational age is counted from the first day of your last menstrual period, not from conception.

You probably already know pregnancy lasts about nine months. But when you ask how many weeks pregnant you are, the answer doesn’t start at the moment of conception. It starts about two weeks before that, on the first day of your last period.

That date — called the last menstrual period, or LMP — is the standard reference point for calculating gestational age. This article walks through how to use it, why an early ultrasound can give a more accurate estimate, and what to do if your cycles don’t fit the textbook pattern.

How Pregnancy Weeks Are Calculated

Gestational age is measured from day 1 of your LMP, even though ovulation and conception typically happen about two weeks later. That means on the day you miss your period, you’re already considered about four weeks pregnant.

The math is simple once you have the date. Your estimated due date (EDD) is calculated by adding 280 days — or 40 weeks — to the first day of your LMP. The NHS due date calculator uses this same logic.

If your cycles are regular and your LMP is known, this method gives a reasonable starting point. Keep in mind that a full-term pregnancy is typically considered anywhere from 37 to 42 weeks, so the EDD is more of an estimate than a deadline.

Why Counting From Your Last Period Can Be Confusing

Many people naturally assume pregnancy starts at conception, so hearing you’re already two weeks along before you even ovulate feels backwards. That mismatch is the main reason the LMP method can seem confusing.

Several factors can also throw off the calculation:

• Conception timing confusion: Sperm can live several days, and ovulation varies, so the exact fertilization date is rarely known. LMP provides a consistent, measurable starting point.
• Irregular cycles: If your cycle is longer or shorter than the typical 28 days, the actual ovulation date may not fall at day 14. An LMP-based estimate can be off by a week or more.
• Spotting mistaken as a period: Light bleeding early in pregnancy can be confused with a normal period, leading to an incorrect LMP date and a skewed gestational age.
• Unknown LMP: If you don’t remember the exact date of your last period — which is common — you can’t rely on this method at all. That’s where ultrasound dating becomes essential.

When any of these situations apply, your provider will typically recommend an early ultrasound to establish or adjust your dates.

What Makes Ultrasound Dating More Accurate

First-trimester ultrasound — performed before 13 weeks and 6 days — is widely considered the most accurate method for confirming gestational age. ACOG notes that even when the LMP is known, an early scan can improve the precision of the estimated due date.

The accuracy does fade over time. In the second trimester, ultrasound dating carries an error margin of about 10–14 days. By the third trimester, the margin can reach up to three weeks, which is why late scans are not used to reassign dates.

Here’s how the different methods compare:

If your dating scan shows a due date that differs from your LMP calculation by more than five days, the scan date typically becomes the official due date. This is why an early ultrasound is so valuable, even if your periods seem regular.

When to Adjust Your Due Date Based on Scan Results

It’s normal for the due date to shift after an ultrasound — sometimes forward, more often backward. The key is knowing when the difference is big enough to matter. Here are the common scenarios:

1. First trimester: If the ultrasound differs from LMP by more than five days, the scan date is usually adopted as the new due date.
2. Second trimester: Your provider will only adjust the due date if the difference is greater than two weeks, because the margin of error is wider.
3. Third trimester: Scans are not used to redate the pregnancy. The error margin is too large, so your LMP or earlier scan remains the reference.

If you have irregular periods or cannot recall your LMP, an early scan is the preferred way to establish gestational age from the start. Your provider will walk you through any adjustments needed.

Understanding Trimesters and How They Relate to Weeks

Once you know how many weeks pregnant you are, you can figure out which trimester you’re in. The three trimesters are defined by standard week ranges and mark different stages of fetal development and maternal changes.

The typical breakdown:

Knowing your trimester helps you prepare for prenatal tests, symptom changes, and discussions with your provider. For a detailed week-by-week guide, What To Expect covers each stage on its pregnancy trimesters weeks page.

The Bottom Line

Knowing how many weeks pregnant you are starts with the first day of your last period. That LMP date gives a quick estimate, but a first-trimester ultrasound is the gold standard for accuracy — especially if your cycles are irregular or your LMP is uncertain. Most providers will combine both methods to set your due date.

Your obstetrician or midwife can review your LMP and any ultrasound results to confirm your dating, and they’ll adjust the due date if the difference exceeds standard thresholds. If you’re unsure about your LMP or missed a period, don’t hesitate to ask for an early scan for a clearer timeline.