What Does an Ultrasound Tell You About Your Due Date? (A Guide)

You've done the math. You’ve used an online calculator based on your Last Menstrual Period (LMP) and you have your due date circled on the calendar. Then, you go for your first prenatal appointment, have an ultrasound, and your doctor gives you a different due date, sometimes by a full week.

This is an extremely common—and confusing—experience. Which date is right? Is the ultrasound more accurate than your own body's tracking?

The simple answer is yes. A first-trimester "dating scan" is considered the gold standard for establishing an accurate Estimated Due Date (EDD). This isn't just a guess; it's a precise medical calculation based on the predictable science of fetal development.

This guide will explain how sonographers determine your due date, why an early ultrasound is so much more accurate than your LMP, and what scans in the second and third trimesters really tell you.

Table of Contents

• Part 1: The Problem with the LMP "Guess"
• Part 2: The Gold Standard — The First Trimester "Dating Scan" (Weeks 8-13)
  • How They Measure: Crown-Rump Length (CRL)
• Part 3: "My Doctor Changed My Due Date!" (YMYL)
• Part 4: What Ultrasounds Tell Us in the 2nd and 3rd Trimesters
  • Why the Shift?
  • What They Measure in Later Scans
• Part 5: Putting It All Together
• Frequently Asked Questions (FAQ)
  • Medical Disclaimer
  • About the Author

Part 1: The Problem with the LMP "Guess"

Before ultrasound, the only way to estimate a due date was to use the first day of your Last Menstrual Period (LMP). The standard calculation, known as Naegele's Rule, adds 40 weeks (280 days) to that date.

However, this method is built on two massive assumptions:

1. It assumes you have a perfect 28-day cycle.
2. It assumes you ovulated on exactly Day 14 of that cycle.

This is simply not the biological reality for most women.

• If your cycle is 35 days long, you likely ovulated around Day 21.
• If your cycle is 24 days long, you likely ovulated around Day 10.
• Even in a regular cycle, stress or illness can delay ovulation by days.

Using the LMP method can be inaccurate by a week or more if your cycle isn't "textbook." This is a significant margin of error when managing a pregnancy.

Part 2: The Gold Standard — The First Trimester "Dating Scan" (Weeks 8-13)

This is why doctors insist on an early ultrasound. The first trimester is the only time in pregnancy when all fetuses grow at a highly predictable, standardized rate.

Your genes haven't taken over yet, so a 9-week-old fetus in one person is almost the exact same size as a 9-week-old fetus in another. This allows for incredibly accurate dating.

How They Measure: Crown-Rump Length (CRL)

During a first-trimester scan (often called the "dating scan"), the sonographer doesn't measure the whole baby. They measure the Crown-Rump Length (CRL).

This is a measurement from the top of the baby's head (the crown) to the bottom of its buttocks (the rump). The legs are not included as they are often curled up.

The CRL measurement is the most accurate predictor of gestational age.

• Accuracy at 8-10 Weeks: The CRL is accurate to within +/- 3 to 5 days.
• Accuracy at 11-13 Weeks: The accuracy is still high, at +/- 5 to 7 days.

This is far more precise than the LMP method, which can be off by weeks.

Part 3: "My Doctor Changed My Due Date!" (YMYL)

This is the most common point of confusion and anxiety. You receive your ultrasound report, and the "Gestational Age (GA)" and "Estimated Due Date (EDD)" are different from your LMP calculation.

Here is the standard medical protocol, supported by the American College of Obstetricians and Gynecologists (ACOG):

1. Your doctor compares your due date from your LMP to the due date from the ultrasound (CRL).
2. If the dates are close (within 5-7 days of each other), your doctor will usually keep your original LMP date.
3. If the dates are significantly different (more than 7 days apart), your doctor will change your official due date to the ultrasound date.

This is not a sign that something is wrong with your baby. It is a positive, corrective measure. It simply means you ovulated earlier or later than you thought, and the ultrasound date provides a much more accurate timeline for monitoring the true health of your pregnancy.

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Part 4: What Ultrasounds Tell Us in the 2nd and 3rd Trimesters

This is where the second major confusion arises. You may have an anatomy scan at 20 weeks where the sonographer says, "The baby is measuring 21 weeks." This does not mean your due date is wrong again.

After the first trimester, ultrasounds are no longer accurate for dating. They are used for monitoring growth.

Why the Shift?

After about 13 weeks, your baby's unique genetics take over. Just like on the outside, some babies are destined to be tall, and some are short. Some are lean, and some are chubby. They no longer grow at a standardized rate.

A 20-week ultrasound that shows a baby "measuring 21 weeks" simply means your baby is in a high percentile for size—it's a big, healthy baby! Your doctor will not change your due date based on this scan.

What They Measure in Later Scans

Instead of the CRL, sonographers measure a combination of four things to estimate fetal weight (EFW):

1. Biparietal Diameter (BPD): The diameter of the baby's head.
2. Head Circumference (HC): The circumference of the baby's head.
3. Abdominal Circumference (AC): The circumference of the baby's belly (this is a key indicator of weight).
4. Femur Length (FL): The length of the thigh bone.

These measurements are plugged into an algorithm to get a weight estimate, which is then compared to the average weight for your official gestational age (the one set in your first trimester). This is how your doctor tracks that the baby is growing properly.

Part 5: Putting It All Together

The most accurate date you will ever get is from a first-trimester (8-13 week) ultrasound measuring the CRL. This is the "gold standard" EDD that will be used for your entire pregnancy.

If you miss this window, a second-trimester scan (up to 22 weeks) is the next most accurate, but the margin of error increases to +/- 10-14 days. Any ultrasound after that is a poor tool for dating and is only used for assessing growth and well-being.

Knowing how this data is gathered empowers you to understand your pregnancy timeline. If you have had an ultrasound and know your baby's measurements, you can use a calculator to see how your EDD is determined.

Your Next Step: Calculate Your Due Date from Your Scan

Have your ultrasound report handy? Use our Ultrasound Due Date Calculator to input your Crown-Rump Length (CRL) or other measurements and see your most accurate due date.

Frequently Asked Questions (FAQ)

Q: Can a 20-week ultrasound be wrong about the due date? A: Yes. A 20-week ultrasound (the anatomy scan) is not the most accurate tool for dating. It has a margin of error of +/- 10 to 14 days. The CRL measurement from a 10-week scan, for example, is far more accurate. Your doctor will always trust the earliest scan over a later one for setting your due date.

Q: My doctor changed my due date by 10 days! Does this mean my baby is growing too fast or too slow? A: No. It means your original due date, based on your Last Menstrual Period (LMP), was incorrect. It simply means you ovulated about 10 days earlier or later than the "Day 14" average. The ultrasound has corrected your timeline, it has not identified a growth problem.

Q: What is a "dating scan" and when is it done? A: A "dating scan" is just another name for the first-trimester ultrasound, performed specifically to determine the gestational age and due date. It is most accurate when performed between 8 and 13 weeks.

Q: Why does my due date from the ultrasound not match my date of conception? A: Remember that "gestational age" (GA) is not the same as "fetal age" (age from conception). Gestational age includes the two weeks before you ovulated. So, when your 10-week ultrasound says you are "10 weeks, 0 days pregnant," your baby's fetal age is only about 8 weeks. Your due date is always calculated as 40 weeks from your LMP, or its ultrasound equivalent.

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Medical Disclaimer

This article is for informational and educational purposes only and is based on general medical guidelines from ACOG. It is not a substitute for professional medical advice. The interpretation of your ultrasound, gestational age, and estimated due date must be done by your qualified healthcare provider or obstetrician.

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.