How to determine pregnancy due date using ultrasound results
Ultrasound (or sonogram) scans are totally painless and safe. They use high-frequency sound waves to get an image of the fetus to check fetal development and the progress of your pregnancy.
The ultrasound can detect the fetus’s heartbeat around the 6th week, and this is one of the ways your health care provider can tell how far along you are in your pregnancy.
The first ultrasound, done between 10 and 14 weeks, is considered the most accurate method of estimating when you conceived.
The Rule of Eights: Due Date by last menstrual period or ultrasound?
You can calculate your due date by last menstrual period (LMP) or by ultrasound, and the two calculation results might be different. How do you determine which is more accurate?
To determine an accurate due date, you can use the “Rule of Eights.”
1. Calculate the difference in due dates. First, determine the difference in days between the LMP and ultrasound due dates. Later, you’ll compare that number to the margin of error, which is calculated by using the Rule of Eights.
2. Calculate the margin of error. The Rule of Eights is based on a pretty consistent 8 percent margin of error found at any gestational age (GA). You can use a simple formula to figure it out:
GA* x 0.08 (8%) = Margin of error
*GA in days as determined by your first ultrasound
3. Compare the results
Now compare the results from the previous two steps:
If the margin of error is bigger than the difference in days between the due dates, use the LMP date as your due date.
If the margin of error is smaller than the difference, use the ultrasound date as your due date.
Can your due date change?
Yes, your estimated due date can change, but it’s nothing to worry about.
An estimated due date can change for several reasons. Your periods may have been irregular, the early ultrasound date could have been off, or perhaps the first ultrasound was performed in the second trimester.
Using a due date calculator can help you plan for the baby’s arrival. Remember that the date can change as you get further along in your pregnancy depending on how your check-ups go. Using Flo’s due date calculators can give you a rough idea of when you’re likely to go into labor.
LMP - Last Menstrual Period (the first day of your last period)
GA - Gestational Age (how many weeks and days pregnant you are)