Ultrasound Accuracy for Pregnancy Issues

Determining a Due Date

It's a question that's obviously on the top of every expectant mother's mind: When am I due?

Evidence suggests that ultrasounds more accurately predict your due date than using your last menstrual period—but only in the first trimester and early second trimester (until roughly 20 weeks). Early ultrasound due dates have a margin of error of roughly 1.2 weeks.

After 20 weeks of pregnancy, 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 4% of women give birth naturally on their due date.

Detecting the Fetal Heartbeat

Ultrasounds can detect a fetal heartbeat in pregnancies beyond 7 to 8 weeks. But during the early weeks of pregnancy, it can be difficult to distinguish an earlier-than-estimated 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 8 weeks into your pregnancy. If your pregnancy has a gestational age of less than 8 weeks (between 6 and 8 weeks), a transvaginal ultrasound (inserting the ultrasound probe into your vagina) is usually needed for accurate results.

Diagnosing a Miscarriage

In the first trimester, it may be hard to differentiate a miscarriage from an early viable pregnancy. 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 and additional testing, such as hCG blood tests.

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.

Diagnosing Birth Defects

Ultrasounds may be used to diagnose birth defects, but they are not always accurate. It's thought that a second-trimester ultrasound, often done between 16 and 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 that suggests a problem when there is nothing wrong.

Second trimester ultrasounds are more likely than first trimester ultrasounds to find fetal anomalies. But even first trimester ultrasounds give important information. A 2016 review found that first trimester ultrasounds were able to find fetal anomalies in around 30% of women at low risk and 60% of those at high risk for having a baby with birth defects.

In other cases, such as Down syndrome, however, ultrasound can't offer a firm diagnosis. Instead, it can show markers associated with a higher risk of various conditions.

Finding Out the Baby's Sex

By the midpoint of pregnancy, an ultrasound can give you a pretty good prediction of your baby's sex (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.

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 their thoughts, especially if they are 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 that can be used to get a better idea.