An anomaly scan is a detailed ultrasound performed around 20 weeks of pregnancy that checks your baby’s organs, bones, and overall development for structural problems. Sometimes called the anatomy scan or mid-pregnancy scan, it’s one of the most thorough prenatal appointments you’ll have, typically lasting 30 to 45 minutes. It’s a routine part of pregnancy care in most countries and is offered to all pregnant people regardless of risk level.
What the Scan Checks
The sonographer works through a systematic checklist covering virtually every part of your baby’s body. This isn’t a quick peek. They’re examining the brain, skull, face (including the lips, nose, and eye sockets), spine in multiple angles, heart chambers and major blood vessels, lungs, diaphragm, stomach, kidneys, bladder, bowel, and all four limbs down to the fingers and toes. They also measure the head circumference, the length of the thigh bone, and the width of the skull to confirm your baby is growing on track for the due date.
The heart gets special attention because congenital heart defects are among the most common structural problems. The sonographer checks the size, position, and angle of the heart, the dividing wall between the chambers, the valves, and the major arteries leaving the heart. They’re looking at blood flow patterns and making sure the plumbing is connected correctly.
Beyond the baby, they also assess the placenta’s position, the amount of amniotic fluid, the umbilical cord’s attachment point, and your cervix. If you want to know the sex, this is usually the scan where it can be identified, though you can choose not to be told.
What It Can and Cannot Detect
The anomaly scan is designed to catch major structural problems like spina bifida, cleft lip, kidney abnormalities, limb differences, and serious heart defects. Detection rates vary by condition. Major heart defects, for example, are picked up in roughly 90% of cases when they’re present, while some brain abnormalities are detected around 70% of the time. No ultrasound catches everything. Some conditions are too subtle to see at 20 weeks, develop later in pregnancy, or simply aren’t visible with this type of imaging.
The scan sometimes picks up what are called “soft markers.” These are minor findings like a bright spot on the heart, a small fluid collection in the kidney, or a cyst in the brain’s choroid plexus. Soft markers are not abnormalities themselves. They’re common, often temporary, and in most cases resolve on their own. Historically they were used to adjust the estimated risk of chromosomal conditions like Down syndrome, but an isolated soft marker in an otherwise normal scan, with no other risk factors, is rarely cause for concern. If one is found, your provider will explain what it means in the context of your overall screening results.
What to Expect During the Appointment
You’ll lie on a couch in a dimly lit room (the low light helps the sonographer see the screen more clearly). They’ll ask you to lift or lower your clothing so they can access your belly, tuck tissue paper along the waistband of your clothes, and apply a cool gel. A handheld probe is then moved across your abdomen to capture images. The pressure can feel mildly uncomfortable but shouldn’t be painful.
You may be asked to drink about 24 ounces of clear fluid an hour before the appointment and avoid emptying your bladder. A full bladder pushes the uterus into a better position for imaging in early scans, though at 20 weeks this matters less and practices vary. Check with your clinic beforehand.
The appointment typically takes about 30 minutes, but it can run longer if your baby is in an awkward position, moving a lot, or if the sonographer needs extra time to get clear views of specific structures. Sometimes you’ll be asked to walk around for a few minutes to encourage the baby to shift, or you may be rescheduled for a follow-up scan.
Why Some Scans Are Harder to Read
Two main factors can make the scan more difficult: your baby’s position and the clarity of the ultrasound image passing through your body. If the baby is facing your spine, for instance, it can be hard to see the face or heart. Overlying arms or legs can cast shadows that block the view of deeper structures. About 4% of scans require a second appointment purely because of fetal positioning.
The other factor is maternal body tissue. Ultrasound waves have to travel through the abdominal wall to reach the baby, and certain types of fatty tissue can scatter and distort the sound beam, producing a fuzzier image. This is often discussed in terms of BMI, but it’s not that straightforward. The type of fat matters more than the amount. A person with a lower BMI can sometimes have poor image quality, while someone with a higher BMI can have perfectly clear images. That said, research has shown that the rate of inadequate images for assessing fetal heart defects rises from about 6% in normal-weight individuals to over 17% in those with a BMI of 30 or higher. If image quality is limited, your sonographer will tell you, and they may recommend a follow-up scan or additional imaging.
What Happens After the Scan
In many settings, the sonographer performing the scan cannot give you a diagnosis on the spot. They collect the images and measurements, which are then reviewed by a specialist. If everything looks normal, you’ll typically be told at the end of the appointment or shortly after. If something needs a closer look, you may be referred for a more detailed scan at a specialist center, sometimes called a fetal medicine unit, where higher-resolution equipment and experienced specialists can get a better picture.
A referral for a follow-up doesn’t automatically mean something is wrong. It can simply mean a particular structure wasn’t seen clearly enough during the first attempt. When a genuine abnormality is found, the specialist team will explain the diagnosis, what it means for your baby, and what options or next steps are available. In some cases, additional testing like amniocentesis or fetal MRI may be offered to get more information.
Most anomaly scans come back completely normal. For the small percentage where a problem is identified, catching it at 20 weeks gives families and medical teams time to plan, whether that means arranging for delivery at a hospital with the right surgical expertise, beginning treatment before birth, or simply preparing emotionally and practically for what lies ahead.