Ultrasound Accuracy - Heart Rate, Due Date, Miscarriage and More

Ultrasounds Have Varying Levels of Accuracy for Different Uses

How accurate is a pregnancy 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 the basics of ultrasound. An ultrasound—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 factors such as the stage of the pregnancy, the quality of the machine, the skill of the practitioner, and the position of the baby in your womb.

Here's some info about the reliability of ultrasound for different pregnancy concerns. When you've finished looking at these topics, you may enjoy looking at our collection of ultrasound photos week by week through pregnancy.

Ultrasound for Determining a Due Date

Female doctor pointing to ultrasound picture with pregnant woman
What is the accuracy of ultrasound for determining your due date, your baby's sex, and more?. 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 (weeks 1 through 12) and early second trimester (until roughly 20 weeks.) During the first 20 weeks of pregnancy, the margin of error is roughly 1.2 weeks.

In your later second (after 20 weeks), and especially in your third trimester, 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. In fact, it's thought that only around four percent of women give birth naturally on their due date.

If you still have questions, learn more about the accuracy of ultrasound in predicting your due date.

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.

An abdominal ultrasound can usually detect a baby's heartbeat if you are at least eight weeks into your pregnancy. If your pregnancy has a gestational age of less than eight weeks (between six and eight weeks), a transvaginal ultrasound (inserting the ultrasound probe into your vagina) is usually needed for accurate results.

It's important to note that hand-held ultrasound or fetal Doppler may not detect the baby's heartbeat until you are 12 weeks along in your pregnancy.

Diagnosing a Miscarriage

How accurate is an ultrasound in 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. Other tools, such as checking your hCG level, are usually used along with an ultrasound

Learn more about the accuracy of ultrasound in diagnosing a miscarriage, and how ultrasound may be used along with other tools to learn if a baby has miscarried.

Ultrasound for Diagnosing Birth Defects

Ultrasounds may be used to diagnose birth defects, but are not always accurate. It's thought that a second trimester ultrasound, often done between 16 an 20 weeks, may detect three out of four major birth defects. On the other hand, it's not uncommon for a woman to have an ultrasound which suggests a problem, when there is nothing wrong. It is important for women having an ultrasound to be aware of this small, but real risk of false positives.

Second trimester ultrasounds are more likely that first trimester ultrasounds to find fetal anomalies, yet even first trimester ultrasounds can sometimes give important information. A 2016 review found that early ultrasounds (first trimester) were able to find fetal anomalies in around 30 percent of women at low risk, and 60 percent of women at an elevated risk of having a baby with a birth defect.

In other cases, such as Down syndrome, however, an ultrasound can't offer a firm diagnosis. Instead, it can show markers associated with a higher risk of various conditions. (Learn more about ultrasound findings and Down syndrome.)

In addition to using ultrasound as part of a combination of tests to evaluate a possible defect, an level II ultrasound or 3D or 4D ultrasound may also be recommended.

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, and you've probably heard stories about people who have prepared for the boy they saw on ultrasound who was actually a girl.

The baby's position and whether or not a boy's testicles have descended can factor into the accuracy of the test.

Many people like to know their baby's sex in order to plan for a nursery. If you decide to learn the sex of your baby, recognize that ultrasound findings are not always accurate. In fact, even babies who have "obviously" appeared to have a penis on ultrasound, may arrive at delivery without one!  There are many reasons why what appears to be one thing on ultrasound may actually be something else. Your obstetrician may offer her thoughts, especially if she is positive based on a good ultrasound look that your baby is one sex or another. Yet still, it's not uncommon for obstetricians to be surprised that their sure fire guess was wrong.

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.

The difficulty with assessing fetal size by ultrasound goes both ways. It's thought that ultrasound is not a reliable tool for evaluating low birth weight, and estimates can also be erroneous when assessing for babies who are large for gestational age (big babies) secondary to gestational diabetes.

If your doctor is concerned that your baby is not growing properly (low birth weight) or is growing too much, there are other tools which can be used to get a better idea.

Sources:

Culliney, K., Parry, G., Brown, J., and C. Crowther. Regimens of Fetal Surveillance of Suspected Large-for-Gestational-Age Fetuses for Improving Health Outcomes. Cochrane Database of Systematic Reviews. 2016. 4:CD011739.

Goto, E. Ultrasound as a Primary Screening Tool for Detecting Low Birthweight Newborns: A Meta-Analysis. Medicine (Baltimore). 2016. 95(35):e4750.

Karim, J., Roberts, N., Salomon, L., and A. Papageorghio. Systematic Review of First Trimester Ultrasound Screening in Detecting Fetal Structural Anomalies and Factors Affecting Screening Performance. Ultrasound in Obstetrics and Gynecology. 2016 Aug 22. (Epub ahead of print).

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