How to Calculate Estimated Date of Birth With Ultrasound

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Many pregnant women do not know exactly when their child was conceived. Even if you know exactly when conception was, many care providers will want to perform an ultrasound to determine a more exact due date. However, since babies grow at different rates, a due date calculated from an ultrasound is never completely accurate.

Things You'll Need

  • Ultrasound machine
  • Take an ultrasound picture of the fetus.

  • Measure the length of the fetus from crown to rump. This is known as the crown-rump length, or CRL.

  • Compare the fetus's CRL to a chart of average size at gestational ages. From this, you can calculate the current gestational age with reasonable accuracy.

  • Subtract the current gestational age from the average pregnancy length of 280 days.

  • Add the difference of days to the date of the ultrasound. This will give you a due date.

Tips & Warnings

  • For highest accuracy, an ultrasound to calculate the due date should be taken during the first trimester. During the first trimester, ultrasounds can predict due date with an accuracy of plus or minus five days.
  • Ultrasounds are not a highly accurate method of predicting due dates, and they should be used with caution. When taken within the first trimester, they are usually accurate within five days. However, ultrasounds in the second trimester are only accurate within 14 days, and third trimester ultrasound are accurate within approximately 21 days. Additionally, even accurate due dates are not exact predictions of when a baby will be born, since the average time for gestation, as well as average gestational growth, both have normal biological variation.

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