An anomaly scan is a detailed ultrasound performed between 18 and 21 weeks of pregnancy that systematically checks your baby’s organs, bones, and overall development for structural problems. Often called the 20-week scan or mid-pregnancy scan, it’s the most thorough ultrasound you’ll have during pregnancy, examining everything from the brain and heart to the spine, limbs, and face. It also evaluates your placenta, blood flow, and the baby’s growth measurements.
What the Scan Checks
The anomaly scan is essentially a head-to-toe survey of your baby’s anatomy. The sonographer works through a long checklist of structures, capturing specific images and measurements as they go. In the head, they look at the brain’s structure, including the fluid-filled spaces, the dividing line between the two hemispheres, and the cerebellum at the back. They measure the head circumference and the distance across the skull.
The face gets its own careful review. The sonographer checks the eye sockets, the nose and nasal bone, both lips, the jaw, and the roof of the mouth. They’re looking for cleft lip, unusual spacing between the eyes, or an underdeveloped jaw, all of which can signal a range of conditions.
The heart is one of the most complex parts of the scan. The sonographer examines the four chambers, the valves between them, and the major blood vessels leading in and out. They confirm the heart is on the correct side of the chest and that blood is flowing in the right direction. Because so many tiny structures need to be visualized, the heart portion often takes the longest.
In the abdomen, the stomach, bladder, kidneys, and the spot where the umbilical cord attaches are all checked. The sonographer confirms there are two arteries in the cord and looks at the bowel for any signs of blockage or dilation. The spine is assessed from three different angles to rule out defects like spina bifida. Each of the twelve long bones in the arms and legs is measured, and the hands and feet are checked to confirm they open normally and have the expected number of fingers and toes.
Beyond the baby, the scan evaluates your placenta’s position (particularly whether it’s covering the cervix), the length of your cervix, and the overall uterine environment.
Conditions It’s Designed to Find
The scan screens for a wide range of structural problems. Major conditions include heart defects such as holes between the chambers, underdeveloped heart structures, and abnormal vessel connections. It also looks for brain abnormalities like excess fluid in the ventricles, spinal defects, kidney malformations, abdominal wall problems, and skeletal conditions affecting bone length or joint position.
Detection rates vary by organ system. Skeletal abnormalities are caught in roughly 58% of cases during a standard mid-trimester scan, while heart defects are detected about 53% of the time. Some specific defects, like a hole between the lower heart chambers or clubfoot, have higher detection rates in the range of 71% to 76%. Spinal anomalies, when detected, are caught with high accuracy in the second trimester, though small defects can occasionally be missed. No ultrasound catches everything, and detection depends heavily on the baby’s position, the mother’s body type, and the skill of the person performing the scan.
Soft Markers vs. Structural Defects
Sometimes the scan reveals findings called “soft markers.” These are subtle features, like a bright spot on the heart, a small cyst in the brain’s lining, mildly dilated kidney drainage, or slightly shortened limb bones. Soft markers are not abnormalities in themselves and appear in many healthy babies. However, they show up more frequently in babies with chromosomal conditions like Down syndrome, so finding one (or several) may prompt your provider to discuss whether additional testing makes sense for you.
The distinction matters because a soft marker on its own rarely means something is wrong. One or more soft markers appear in at least 50% of babies who do have a chromosomal condition, but they also appear regularly in babies who are completely healthy. Your provider weighs these findings against your age, prior screening results, and overall risk profile before recommending any next steps.
What the Appointment Is Like
The scan is a standard abdominal ultrasound using 2D imaging, which remains the primary tool for medical anomaly screening. A sonographer applies gel to your abdomen and moves a handheld probe to capture images. You can typically watch the screen during the exam. The appointment generally takes 20 to 45 minutes, though it can run longer if the baby is in an awkward position or if the sonographer needs to get specific views of the heart or spine.
You don’t usually need a full bladder at this stage, unlike earlier pregnancy ultrasounds. Wearing a two-piece outfit makes access easier. If the baby isn’t cooperating, you may be asked to walk around briefly or shift positions so the baby moves.
Most parents also want to know the baby’s sex at this appointment. Reviewing the external genitalia is a standard part of the anatomical checklist, and most providers will share the sex if you want to know. Let the sonographer know your preference at the start so they can avoid accidental reveals if you’d rather be surprised.
Getting Your Results
In most cases, the sonographer or a clinician will give you preliminary findings during the appointment itself. If everything looks normal, you’ll typically be told right away. You may also receive printed or digital images from the scan.
If something unexpected is found, the process changes. The clinician will explain what was seen, what it might mean, and whether you need a follow-up scan, a referral to a fetal medicine specialist, or additional testing. Follow-up might include a more detailed ultrasound at a specialist center, or it could involve genetic counseling and the option of an amniocentesis (where a small sample of amniotic fluid is tested for chromosomal conditions). If a concern is flagged early, chorionic villus sampling from the placenta is sometimes offered instead.
Not every flagged finding turns into a diagnosis. Some findings require monitoring with repeat scans later in pregnancy, and a number of them resolve on their own. The waiting period between a flagged result and a definitive answer can be stressful, but having a clear follow-up plan helps.
What the Scan Cannot Do
The anomaly scan is a powerful screening tool, but it has real limits. It cannot detect every birth defect. Some conditions, particularly certain heart defects and internal organ problems, only become visible later in pregnancy or after birth. Abdominal abnormalities have the lowest detection rate in the mid-trimester, at roughly 28%. Conditions that affect function rather than structure, like hearing loss, learning disabilities, or many genetic syndromes, won’t show up on ultrasound at all.
The scan also doesn’t replace genetic testing. It can raise or lower suspicion for chromosomal conditions, but a definitive chromosomal diagnosis requires a blood-based screening test or an invasive procedure like amniocentesis. Think of the anomaly scan as one piece of a larger picture that includes your earlier screening results, your medical history, and any additional tests your provider recommends.