How to Calculate Your Due Date: 3 Methods (With Examples)
Learn how to calculate your pregnancy due date using 3 proven methods. We provide clear examples for Naegele's Rule (LMP), conception date, and explain why ultrasound is the most accurate.
After the excitement of a positive pregnancy test, the very next question that pops into your mind is almost always: "When is my baby due?"
Knowing your estimated due date (EDD) is more than just a date to circle on your calendar. It's a crucial piece of information that helps your doctor or midwife track your baby's growth, schedule essential tests and screenings, and manage your prenatal care.
While only about 5% of babies are born on their exact due date, this estimate is the anchor for your entire pregnancy journey.
This guide will walk you through the three primary methods used to calculate your due date, complete with step-by-step examples, and explain why the "easiest" method is also the most recommended.
Table of Contents
- What Exactly is an Estimated Due Date (EDD)?
- Method 1: Using Your Last Menstrual Period (Naegele's Rule)
- Method 2: Using Your Date of Conception
- Method 3: The First-Trimester Ultrasound (The "Gold Standard")
- The Easiest & Most Reliable Way: A Due Date Calculator
- Frequently Asked Questions (FAQ)
What Exactly is an Estimated Due Date (EDD)?
Your estimated due date (EDD) is the projected date your baby will be born. A full-term pregnancy lasts, on average, 40 weeks (280 days).
But 40 weeks from when?
This is where things can get confusing. The 40-week countdown doesn't start from the day you conceived. It starts from the first day of your last menstrual period (LMP).
Why Does Pregnancy "Officially" Start Before Conception?
It sounds strange, but for calculation purposes, your pregnancy includes about two weeks before you even conceived.
- Week 1 & 2: Your body is preparing for ovulation (releasing an egg). You aren't actually pregnant yet.
- Conception (around Week 2-3): You ovulate, and the egg is fertilized. This is the true "start" of your baby's development.
- Gestational Age: When your doctor says you are "6 weeks pregnant," they are using the gestational age, which is counted from your LMP. The fetal age (the baby's actual age) is about two weeks less.
Because most women know the date of their last period but not the exact date of conception, using the LMP has become the universal standard for calculating a 40-week pregnancy.
Method 1: Using Your Last Menstrual Period (Naegele's Rule)
This is the most common and traditional way to calculate your due date. It's the method most pregnancy wheels and online calculators are based on. It assumes you have a regular 28-day menstrual cycle.
The formula is known as Naegele's Rule:
- Start with the first day of your Last Menstrual Period (LMP).
- Subtract 3 months.
- Add 7 days.
- Add 1 year.
Let's walk through two clear examples.
Example 1: Straightforward Calculation
Let's say the first day of your LMP was June 10, 2025.
- Start: June 10, 2025
- Subtract 3 months: March 10, 2025
- Add 7 days: March 17, 2025
- Add 1 year: March 17, 2026
Your Estimated Due Date is: March 17, 2026
Example 2: A "Tricky" Calculation (Crossing the New Year)
What if your LMP was early in the year? Let's say your LMP was February 15, 2025.
- Start: February 15, 2025
- Subtract 3 months: November 15, 2024 (You go back into the previous year)
- Add 7 days: November 22, 2024
- Add 1 year: November 22, 2025
Your Estimated Due Date is: November 22, 2025
The Big Limitation of the LMP Method
Naegele's Rule is a great estimate, but it has one major flaw: it assumes everyone has a 28-day cycle and ovulates on exactly Day 14.
This is not the case for many women. If your cycles are longer (e.g., 35 days) or shorter (e.g., 21 days), or if you have irregular periods, the LMP method can be off by a week or more.
Method 2: Using Your Date of Conception
This method is more biologically accurate if you know your exact date of ovulation.
Women who have been using Ovulation Predictor Kits (OPKs), charting their Basal Body Temperature (BBT), or who conceived via assisted reproductive technology (like IVF) may know this date.
The formula is simple:
- Start with your exact date of conception (ovulation).
- Add 266 days (which is 38 weeks).
Why 266 days? Because it's the 40 weeks (280 days) of a full pregnancy minus the 2 weeks from your LMP to ovulation.
Example:
Let's say you were tracking ovulation and know you conceived on July 1, 2025.
- Start: July 1, 2025
- Add 266 days: ...this is where it gets tricky to do by hand.
Adding 266 days from July 1, 2025 brings you to... March 14, 2026.
Your Estimated Due Date is: March 14, 2026
The Challenge of This Method
As you can see, the math is cumbersome, and the vast majority of women cannot pinpoint their exact date of conception. Even if you only had intercourse once during your fertile window, sperm can live for up to 5 days, making the "conception date" a small window rather than a single day.
Method 3: The First-Trimester Ultrasound (The "Gold Standard")
This is, by far, the most accurate way to determine your baby's due date. Your doctor or midwife will schedule this ultrasound, often called a "dating scan," during your first prenatal visit (typically between 8 and 13 weeks).
How It Works
Instead of guessing based on your cycle, the sonographer performs a simple, non-invasive measurement of your baby.
- Crown-Rump Length (CRL): The technician will measure your baby from the top of its head (the crown) to the bottom of its buttocks (the rump).
- Why It's So Accurate: In the first trimester, all embryos and fetuses grow at a very consistent and predictable rate. The CRL measurement can date a pregnancy with an accuracy of +/- 5 to 7 days.
After 13 weeks, babies start to grow at more individualized rates, so ultrasounds in the second or third trimester are less reliable for dating.
What If My Ultrasound Date and LMP Date are Different?
This is extremely common!
Your doctor will compare your LMP-calculated due date with your ultrasound-calculated due date.
- If the dates are close (within 5-7 days), they will likely stick with your original LMP date.
- If the dates are significantly different (more than 7-10 days), your doctor will almost always change your due date to the new, more accurate date provided by the ultrasound. This is especially true for women with irregular cycles.
The Easiest & Most Reliable Way: A Due Date Calculator
As you can see, the manual calculations can be confusing (especially Naegele's Rule) or simply not possible (if you don't know your conception date).
An online due date calculator combines all these complex methods into a simple, instant tool. It can handle the tricky "new year" math for you and even provide your due date based on conception date, IVF, or an ultrasound.
It's the perfect first step to take after a positive test.
Your Next Step: Find Your Due Date in Seconds
Skip the confusing math and get an instant, reliable estimate. Our calculator uses the standard LMP method, conception date, and even IVF to help you pinpoint your date.
Frequently Asked Questions (FAQ)
Q: Can my due date change? A: Yes. It's very common for your due date to be adjusted, especially after your first-trimester ultrasound. This new date is considered more accurate and will be the one your doctor uses for the rest of your pregnancy.
Q: What if I have an irregular cycle? A: If your cycles are irregular, the LMP method (Naegele's Rule) will be inaccurate. In this case, your doctor will rely almost exclusively on your first-trimester ultrasound (the CRL measurement) to establish your due date.
Q: I know my due date. Why does my doctor talk about "weeks"? A: Pregnancy is managed in weeks, not months. This is far more precise. When your doctor says "you are 12 weeks pregnant," it's a specific milestone. A full-term pregnancy is 40 weeks, and key tests are scheduled for specific weeks (e.g., the anatomy scan around 20 weeks).
Q: Does getting a C-section change my due date? A: No. Your estimated due date is the date your baby is projected to be full-term (40 weeks). A C-section is a delivery method. If you have a planned C-section, it is typically scheduled for the 39th week, just before your official due date, but it doesn't change the due date itself.
Medical Disclaimer
The information in this article is for educational and informational purposes only and is not intended as medical advice. Any due date calculation is an estimate. Your true due date will be confirmed by your healthcare provider. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding your pregnancy.
About the Author
Abhilasha Mishra is a health and wellness writer specializing in women's health, fertility, and pregnancy. With a passion for empowering individuals through evidence-based information, she writes to make complex health topics accessible and actionable.