An anomaly scan is a detailed ultrasound performed between 18 and 22 weeks of pregnancy that checks your baby’s physical development from head to toe. Often called the anatomy scan or 20-week scan, it’s the most thorough ultrasound you’ll have during pregnancy. The appointment typically lasts around 45 minutes and evaluates the brain, heart, spine, kidneys, limbs, and other major structures to look for any developmental concerns.
What the Scan Checks
The anomaly scan follows a systematic checklist that covers nearly every part of your baby’s body. The sonographer examines the brain from multiple angles, looking at the skull shape, the fluid-filled spaces inside the brain, and the cerebellum (the structure at the back of the brain that coordinates movement). They also check your baby’s face, including the eye sockets, nose, and mouth, to look for conditions like cleft lip.
The heart gets special attention. The sonographer checks its size, position, and rhythm, then looks at all four chambers and the major blood vessels carrying blood in and out. Heart defects are among the most common birth differences, so this portion of the scan is particularly detailed.
Beyond the brain and heart, the scan covers the chest and lungs, the diaphragm (the muscle separating the chest from the abdomen), the stomach, the kidneys and bladder, and the spot where the umbilical cord attaches to the belly. The spine is imaged along its full length. Each of the twelve long bones in the arms and legs is measured individually, and the hands and feet are visualized. All of this gives a comprehensive picture of how your baby’s body is forming.
How Accurate Is the Scan?
Detection rates vary depending on the condition. A population-based study spanning 30 years found that the scan picks up spina bifida about 91% of the time and identifies severe brain conditions like anencephaly at 98%. For heart defects, accuracy depends on the specific problem: hypoplastic left heart (where the left side of the heart is underdeveloped) is detected roughly 96% of the time, while transposition of the great arteries (where two major heart vessels are switched) is caught about 82% of the time.
Some conditions are harder to spot. Limb reduction defects, where part of an arm or leg doesn’t fully develop, are detected about 54% of the time. Diaphragmatic hernia, where abdominal organs push up into the chest cavity, is identified around 71% of the time. Conditions involving missing kidneys or abdominal wall openings are caught at or near 100%. No ultrasound catches everything, but the anomaly scan is the single most effective screening tool for structural differences during pregnancy.
What Happens During the Appointment
The sonographer applies gel to your abdomen and moves a handheld device called a transducer across your belly to capture images. You’ll lie on your back for most of it. If your baby is in an awkward position that blocks certain views, you may be asked to shift around or drink something sweet to encourage movement. Once the sonographer has all the images and measurements they need, they wipe off the gel and you’re done.
Some clinics ask you to arrive with a partially full bladder, which can help produce clearer images early in the scan. Your provider will give you specific instructions when scheduling the appointment. There’s no pain involved, and the ultrasound itself poses no known risk to you or your baby. Safety guidelines recommend keeping exposure time and intensity within established limits, and a standard 45-minute scan falls well within those boundaries.
Finding Out the Sex
The anomaly scan is when many parents learn whether they’re having a boy or girl. By the second trimester, sonographers can determine fetal sex with virtually 100% accuracy. In a study of over 200 second-trimester scans, correct sex assignment was achieved in every case where a prediction was made. The rare exceptions involved high maternal BMI, uterine fibroids, or the baby being in a position that blocked the view. If you don’t want to know, let the sonographer know at the start of your appointment.
What Happens If Something Looks Abnormal
Most anomaly scans come back normal. When the sonographer spots a potential concern, the images are reviewed by a specialist and you’ll typically be called back for a more detailed follow-up ultrasound. Depending on what’s found, your provider may recommend additional diagnostic testing.
Amniocentesis, which involves collecting a small amount of the fluid surrounding your baby, can test for chromosomal conditions like Down syndrome and genetic conditions like cystic fibrosis. It also measures specific protein levels that can indicate neural tube defects such as spina bifida. This test is usually done between 15 and 18 weeks, so if concerns arise later, other testing options may be discussed.
Some findings on the scan are “soft markers,” minor variations that on their own are harmless but in combination with other factors might slightly raise the chance of a chromosomal condition. Your provider will walk you through what was seen, what it means in context, and whether any next steps are recommended. In cases where a significant structural issue is confirmed, you may be referred to a maternal-fetal medicine specialist, and your care team may suggest planning your delivery at a hospital equipped to provide specialized newborn care.
Why the Timing Matters
The 18-to-22-week window isn’t arbitrary. Earlier in pregnancy, many structures are simply too small to evaluate clearly. By 18 weeks, the baby’s organs have developed enough to be visible on ultrasound, and the baby is still small enough relative to the uterus that the sonographer can get good views. Wait much longer and the baby’s increasing size and more cramped positioning can make certain measurements harder to obtain. This window balances image quality with developmental maturity, giving the clearest possible picture of how your baby is growing.