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% 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 technician, 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. 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. 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. 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. 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. 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. 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.
