Ultrasound Accuracy - Heart Rate, Miscarriage Diagnosis and More

Ultrasounds Have Varying Levels of Accuracy for Different Uses

How accurate is ultrasound? You may be wondering this about your due date, your baby's gender or more serious things like a miscarriage diagnosis.

First, let's quickly go over what an ultrasound is. This test - also known as a sonogram - uses sound waves to produce a picture of your baby in the womb. These pictures show up on a computer screen at your bedside during the test.

Ultrasound is an amazing tool for tracking the development of a pregnancy, and it gives doctors a lot of useful information for providing optimal prenatal care. It also, of course, gives you the first glimpse of your baby!

But ultrasounds are not 100 percent reliable for everything they measure. The accuracy of an ultrasound test can vary based on a few factors:

  • the stage of the pregnancy
  • the quality of the machine
  • the skill of the practitioner¬†

Here's some info about the reliability of ultrasound for different pregnancy concerns.

Ultrasound for Determining a Due Date

Female doctor pointing to ultrasound picture with pregnant woman
Blend Images - Jose Luis Pelaez Inc/Brand X Pictures/Getty Images

It's a question that's obviously on the top of every expectant mother's mind: When am I due? The evidence suggests that ultrasound is more accurate than the last menstrual period date in predicting when the baby will be born - but only in the first trimester. After that, your estimated due date shouldn't change based on an ultrasound because it will be less accurate. And remember: It's an estimated due date; the vast majority of women don't deliver their babies the day they're due.

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Detecting the Fetal Heartbeat

An ultrasound should always detect the baby's heartbeat in a pregnancy that is beyond seven to eight weeks in gestational age. But during the early part of the first trimester, it can be difficult to distinguish an earlier-than-estimated but viable pregnancy from a missed miscarriage. For that reason, it usually takes two ultrasounds conducted several days apart to confirm or rule out a miscarriage at this stage.

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Diagnosing a Miscarriage

As we said above, in the first trimester it may be hard to differentiate a miscarriage from an early viable pregnancy based on a single ultrasound. That's why doctors often need two consecutive scans to diagnose miscarriage. But sometimes it is possible to confirm a miscarriage based on a single ultrasound.

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Ultrasound for Diagnosing Birth Defects

Certain birth defects can be diagnosed fairly reliably from ultrasound results. In other cases, however, an ultrasound can't offer a firm diagnosis. Instead, it can show markers associated with a higher risk of various conditions.

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Finding Out the Baby's Gender

By the midpoint of pregnancy, an ultrasound can give you a pretty good prediction of your baby's gender (if you want to know). But it is possible for the ultrasound prediction to be incorrect. The baby's position and whether or not a boy's testicles have descended can factor into the accuracy of the test.

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Predicting the Baby's Size

Research suggests that ultrasound predictions of a baby's size are not very reliable. In fact, it's possible for the prediction to be off by multiple pounds.

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