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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.
To calculate your due date using the Flo calculator, follow these steps:
1. Insert the date when you had your ultrasound in the “Date of ultrasound” field.
2. Insert the week and day of your pregnancy that was estimated by the ultrasound — this is the gestational age of the fetus or how far you are in your pregnancy.
Now, the calculator will determine your due date by adding the remaining length of a typical pregnancy (in days) to how far along you are in your pregnancy (current gestational age).
3. Click the “Calculate my due date” button and check your results.
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:
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.
Notes:
LMP - Last Menstrual Period (the first day of your last period)
GA - Gestational Age (how many weeks and days pregnant you are)