How Prenatal Ultrasounds Work and What They Show

Prenatal ultrasounds are among the most recognizable moments of pregnancy. They offer the first visual confirmation that your pregnancy is progressing as expected and provide detailed information about your baby’s development, position, and health. Understanding how ultrasounds work and what doctors are looking for at each scan can help you feel more prepared during these appointments.

How Prenatal Ultrasounds Work

Ultrasound imaging relies on sound waves at frequencies too high for the human ear to detect, typically between 2 million and 18 million cycles per second, or hertz. A handheld device called a transducer emits these sound waves through the skin into the uterus. When the sound waves strike your baby, the placenta, amniotic fluid, or the uterine wall, they bounce back toward the transducer. The transducer captures these echoes and sends them to a computer, which translates the returning sound waves into images on a screen.

This technology, called the Doppler effect, allows ultrasound machines to create real-time images and even measure blood flow. Unlike X-rays, ultrasound does not use radiation, making it considered safe for use during pregnancy. The FDA has found no evidence that ultrasound causes harm to a developing fetus when used at standard diagnostic levels, though medical organizations recommend using ultrasound only when there is a medical reason and keeping exposure as brief as practical.

Types of Prenatal Ultrasounds

Transvaginal Ultrasounds

A transvaginal ultrasound places a thin transducer covered with a sterile sheath directly inside the vagina. Because the transducer sits closer to the uterus, these scans produce higher-resolution images and are the standard approach for early pregnancy evaluation. Transvaginal ultrasounds typically occur between weeks 8 and 10 and are used to confirm pregnancy viability, check for a heartbeat, and establish a due date based on measurements of the developing embryo.

Transabdominal Ultrasounds

A transabdominal ultrasound places the transducer on your belly over a layer of ultrasound gel. This approach works best after about 10 to 12 weeks of pregnancy, when the growing uterus sits higher in the abdomen and can be seen more clearly from the outside. Transabdominal ultrasounds are used throughout the second and third trimesters and allow the technician to view the entire uterus, placenta, and surrounding structures in one image.

Three-Dimensional and Four-Dimensional Ultrasounds

Three-dimensional (3D) ultrasounds take multiple 2D images from different angles and reconstruct them into a 3D model of your baby. Four-dimensional (4D) ultrasounds add the dimension of time, showing movement in real time. These scans are not routine for medical evaluation and are often elective, done primarily for keepsake images or when the standard approach has not provided enough detail to answer a clinical question.

The First Trimester Ultrasound

The first prenatal ultrasound typically occurs between weeks 8 and 10 and answers the most fundamental questions: Is the pregnancy developing inside the uterus (not in a fallopian tube)? Is there a fetal heartbeat? How many babies are there? The ultrasound technician measures the crown-rump length (CRL), the distance from the top of the baby’s head to the base of the spine, to establish or refine your due date. This measurement is most accurate at this stage, with a margin of error of only a few days.

If this is your first appointment with an obstetrician or midwife, the provider may also measure the nuchal translucency (NT), a small pocket of fluid at the back of your baby’s neck, as part of first-trimester screening for chromosomal abnormalities like Down syndrome. The NT measurement, combined with blood tests and maternal age, helps calculate your risk. However, a normal measurement does not guarantee a normal pregnancy, nor does a slightly enlarged measurement definitively indicate a problem; further testing would be needed to clarify.

The Anatomy Scan at 18 to 22 Weeks

The mid-pregnancy ultrasound, sometimes called the anatomy scan or level 2 ultrasound, occurs between 18 and 22 weeks and is considered the most detailed survey of your baby’s physical development. The technician systematically evaluates the brain, spine, heart, lungs, kidneys, bladder, arms, legs, hands, and feet. They check the position of the placenta and measure the amniotic fluid volume. The sonographer also confirms your baby’s sex if the position allows clear visualization of the genitals and if you have chosen to learn it.

Measurements at this scan include the femur length (thighbone), head circumference, and abdominal circumference—markers used to confirm dates, assess growth, and identify pregnancies at risk for certain complications. The heart receives particular attention: the technician counts the chambers, checks the position, and evaluates the direction of blood flow through the four chambers and the major vessels leading away from the heart. Though a detailed heart scan (fetal echocardiography) is not routine, it may be recommended if there is a family history of heart defects or if the standard ultrasound raises concerns.

Third Trimester Scans and Monitoring

Scans in the third trimester, typically at 28 to 32 weeks, assess fetal growth and development as delivery approaches. The technician measures the baby, checks the amniotic fluid volume, confirms placental location, and evaluates blood flow in the umbilical cord using Doppler ultrasound. If growth appears slower than expected for the stage of pregnancy, additional measurements and monitoring may be recommended. Depending on your pregnancy history and any complications that have arisen, you may receive additional scans or nonstress tests (NSTs), which record the baby’s heart rate and movements over 20 to 30 minutes.

In the final weeks before delivery, ultrasound can confirm your baby’s position (head down or breach), predict birth weight, and assess whether labor is medically urgent if concerns arise about placental function or fetal well-being.

Understanding Ultrasound Measurements and Markers

Every measurement on a prenatal ultrasound is compared to standard ranges for gestational age. Small variations between babies are normal; growth charts account for individual differences. A measurement slightly below or above the average does not necessarily indicate a problem. Doctors look for trends across multiple scans and for how measurements track against your due date.

When an ultrasound identifies a soft marker (a feature that appears slightly unusual but is often benign and resolves by birth), the technician documents it and may recommend follow-up scans or additional testing to provide reassurance. Examples include an echogenic focus (a bright spot in the heart), echogenic bowel (bowel that appears brighter than normal), or mild renal pelvis dilation. Most soft markers turn out to have no clinical significance, though some warrant further evaluation to rule out chromosomal conditions or other complications.

Limitations and Accuracy of Prenatal Ultrasound

Ultrasound is a powerful diagnostic tool, but it cannot detect every abnormality or condition. The quality of the images depends on the position of your baby, the amount of amniotic fluid, the thickness of your abdominal tissue, and the experience of the technician and radiologist interpreting the scan. Some conditions may not be visible on ultrasound at all, or symptoms may not appear until after birth. Additionally, ultrasound cannot diagnose genetic or chromosomal conditions with certainty—it can only identify physical markers that may raise concern and prompt further testing such as amniocentesis.

If your ultrasound identifies a potential concern, your provider will discuss the finding, explain what it might mean, and outline options for further evaluation or management. A single abnormal finding does not define the outcome of your pregnancy, and additional testing is often recommended before drawing conclusions.