Fetal anatomy refers to both the physical structures of a developing baby and, more commonly, the ultrasound scan used to examine those structures during pregnancy. Known as the anatomy scan or anomaly scan, this examination typically happens between 18 and 22 weeks of pregnancy. It’s a detailed look at your baby’s organs, bones, brain, and heart to confirm everything is developing on track and to catch potential problems before birth.
What the Anatomy Scan Checks
The scan is a systematic review of your baby’s entire body. The sonographer works through a checklist of structures, capturing images of each one. The major areas include the brain and skull, face, spine, heart, chest and lungs, stomach, kidneys, bladder, limbs, and the umbilical cord where it connects to the abdomen. Beyond the baby itself, the scan evaluates the placenta’s position and confirms there’s a healthy amount of amniotic fluid surrounding the baby.
This isn’t a quick glance. The sonographer needs to visualize each structure from specific angles, and some parts of the body require multiple views. If the baby is in an awkward position, you may be asked to walk around or shift onto your side so the sonographer can get a clearer look.
How the Baby’s Brain Is Evaluated
Several specific brain structures are checked during the scan. The sonographer looks at the lateral ventricles, which are fluid-filled spaces inside the brain, to make sure they aren’t enlarged. The cerebellum, the small structure at the back of the brain responsible for coordination, is measured. The cisterna magna, a pocket of fluid behind the cerebellum, is also assessed because abnormal size there can signal developmental issues. The midline of the brain is examined to confirm the two hemispheres are dividing properly, and the choroid plexus, which produces spinal fluid, is visible on each side.
The Heart Gets Special Attention
Heart defects are among the most common birth differences, so the cardiac portion of the scan is particularly thorough. The sonographer captures what’s called a four-chamber view, showing both upper chambers (atria) and both lower chambers (ventricles) of the heart. They also look at the outflow tracts, the pathways where blood leaves the heart through the two major blood vessels: the aorta and the pulmonary artery. A three-vessel view confirms these vessels and the large vein returning blood from the body are the right size and in the right positions relative to each other.
Color Doppler, which shows blood flow in real time on the screen, is often used during this part of the exam to check that blood is moving through the heart in the correct direction and that the valves are functioning properly.
Abdomen, Kidneys, and Spine
In the abdomen, the sonographer confirms that the stomach is on the left side and the liver on the right, which verifies the baby’s organs are oriented correctly. The kidneys are checked on both sides, and the bladder should contain visible fluid, a sign the kidneys are already filtering. The umbilical cord insertion point on the belly is examined to rule out abdominal wall defects.
The spine is traced from the neck down to the tailbone, looking for any gaps or irregularities that might suggest a neural tube defect like spina bifida. The skin covering the spine should appear intact along its full length.
Growth Measurements and Gestational Age
Four key measurements are taken during the scan to assess your baby’s size and confirm gestational age. These are considered the gold standard for tracking fetal growth:
- Biparietal diameter (BPD): the width of the baby’s head, measured from one side of the skull to the other
- Head circumference (HC): the total distance around the baby’s head
- Abdominal circumference (AC): the measurement around the baby’s belly, which reflects nutrition and liver size
- Femur length (FL): the length of the thighbone, the longest bone in the body
These numbers are compared against established growth charts for the baby’s gestational age. Together, they give a more reliable picture than any single measurement alone. If there’s a significant difference between the measurements and the expected range, your provider may recommend follow-up scans to monitor growth over time.
Placenta Position and Cervical Length
The placenta’s location matters because if it’s sitting low in the uterus, covering or near the cervix, it can affect delivery planning. Most low-lying placentas move upward as the uterus grows, so an early finding doesn’t necessarily mean a problem at birth, but it will be rechecked later in pregnancy.
Cervical length may also be measured, particularly if you have risk factors for preterm birth. A cervix shorter than 25 millimeters before 28 weeks is considered short and is associated with a higher chance of early delivery. This measurement is taken with a transvaginal ultrasound probe, which is more accurate than an external measurement. For most women, cervical length at this stage is well above that threshold.
What the Scan Can and Cannot Detect
The anatomy scan is good at identifying structural differences, but detection rates vary depending on the body system involved. Skeletal abnormalities like clubfoot are detected roughly 58% of the time at mid-trimester scans. Heart defects are found in about 53% of cases, with some types easier to spot than others. Conditions where the heart is severely underdeveloped are identified nearly 100% of the time, while more subtle defects involving specific valves or vessel arrangements are caught less reliably, sometimes in only a third of cases.
Neural tube defects like spina bifida are detected about 67% of the time. Cleft lip is identified in roughly 64% of cases, though cleft palate alone (without a visible lip gap) is harder to see. Kidney and urinary tract abnormalities are caught about 45% of the time. Brain abnormalities overall have a detection rate around 32%, partly because some conditions don’t become visible until later in pregnancy.
These numbers mean the anatomy scan is a powerful screening tool but not a guarantee. A normal scan is reassuring, and an abnormal finding gives you and your provider time to plan. Some conditions flagged during the scan resolve on their own. Others lead to additional testing, such as a targeted scan by a specialist or genetic testing, to get a clearer picture.
What to Expect During the Appointment
The scan typically takes 30 to 45 minutes, though it can run longer if the baby isn’t cooperating with positioning. You’ll lie on an exam table while a sonographer applies gel to your abdomen and moves a handheld probe across your skin. The images appear on a monitor in real time, and you can usually watch along.
You don’t need to do much to prepare. Some clinics ask you to drink water beforehand so your bladder is partially full, which can help create a clearer image early in the exam. Wearing a two-piece outfit makes it easier to expose your belly without changing into a gown. This is also the scan where you can typically find out the baby’s sex, if you want to know.