The 20-week anatomy scan is the most detailed ultrasound of your pregnancy, lasting around 45 minutes as a sonographer systematically checks your baby’s organs, bones, brain, and growth. It’s typically scheduled between 18 and 22 weeks and is the one ultrasound that virtually every pregnant person receives. Here’s what actually happens during the appointment and what the sonographer is looking for.
Before the Appointment
There’s very little preparation required. Some providers suggest eating a meal or snack beforehand, since a baby that’s actively moving makes it easier for the sonographer to capture all the views they need. You’ll be asked to slightly lower your pants and lift your shirt so the sonographer can apply gel and move the transducer across your belly. Unlike early pregnancy ultrasounds, a full bladder isn’t usually required at this stage, though your provider’s instructions may vary.
What the Sonographer Checks
The scan follows a structured checklist that covers your baby from head to toe. It’s not a casual look around. The sonographer needs specific views of specific structures, and they’ll spend time adjusting the transducer angle to capture each one clearly.
Head and Brain
The sonographer examines the skull for normal shape and completeness, then moves to the brain’s internal structures: the fluid-filled ventricles, the cerebellum (which coordinates movement), and the cisterna magna (a fluid space at the back of the brain). They measure the width of the head from side to side and the circumference around it. These brain views can flag conditions like excess fluid in the ventricles or structural differences in the back of the skull.
Face and Spine
Your baby’s profile, eye sockets, and upper lip are all examined. The upper lip view specifically screens for cleft lip. The spine is checked both lengthwise and in cross-section from the neck down to the tailbone. In the lengthwise view, a healthy spine looks like parallel railroad tracks that widen toward the head and taper at the base. In cross-section, the sonographer confirms that each vertebra’s three growth centers are properly aligned.
Heart
The heart gets some of the most careful attention. The sonographer checks the heart rate and rhythm, then captures a four-chamber view showing both upper and lower chambers. They also look at the outflow tracts, the two major blood vessels that carry blood out of the heart. Heart defects are among the most common congenital conditions, and while not all can be detected at this stage, the four-chamber and outflow tract views catch many of them.
Chest and Abdomen
The lungs and diaphragm are checked for normal appearance. In the abdomen, the sonographer looks for the stomach (which should be filled with swallowed amniotic fluid), the liver, both kidneys, the bladder, and the abdominal wall where the umbilical cord inserts. Each organ’s presence and position confirms that major systems are developing on track.
Arms and Legs
All four limbs are examined individually. The sonographer identifies each long bone: the upper arm bone, the two forearm bones, the thigh bone, and the two lower leg bones. They also look at the hands and feet, checking bone appearance and joint movement. The thigh bone length is one of the key growth measurements taken during the scan.
Growth Measurements
Four standard measurements, called fetal biometry, tell your provider whether your baby is growing appropriately for gestational age. These are the head circumference, the distance across the head (biparietal diameter), the abdominal circumference, and the femur (thigh bone) length. Together, they also provide an estimated fetal weight. Your provider compares these numbers to established growth curves. A measurement that falls outside the expected range doesn’t necessarily mean something is wrong, but it may prompt closer monitoring or a follow-up scan.
Placenta, Fluid, and Cervix
The scan isn’t only about the baby. Your provider also needs information about the environment supporting the pregnancy.
The sonographer notes where the placenta is attached, whether it’s on the front wall (anterior), back wall (posterior), top (fundal), or sides of the uterus. Crucially, they measure how far the placenta sits from the cervix. A placenta that partially or fully covers the cervix, called placenta previa, often resolves as the uterus grows but requires follow-up imaging later in pregnancy.
Amniotic fluid levels are measured by dividing the uterus into four quadrants and finding the deepest pocket of fluid in each one. Those four numbers are added together to produce an amniotic fluid index. At 20 weeks, a normal index centers around 14 centimeters. A measurement below 5 centimeters suggests too little fluid, while anything above 24 to 25 centimeters suggests too much. Both extremes can affect the baby’s development and warrant further evaluation.
Many providers also measure cervical length at this scan. A cervix shorter than about 25 millimeters before 24 weeks raises the risk of preterm birth and may lead to treatment options like progesterone supplementation or, in some cases, a stitch to reinforce the cervix. A measurement between 25 and 29 millimeters is borderline and typically means additional monitoring with repeat ultrasounds.
What “Soft Markers” Mean
Sometimes the sonographer spots minor findings called soft markers. These are not abnormalities. They’re small variations, like a bright spot on the heart (echogenic intracardiac focus) or tiny fluid-filled cysts in the brain’s choroid plexus, that occur in perfectly healthy babies but appear at slightly higher rates in certain chromosomal conditions.
Choroid plexus cysts, for example, show up in up to 3.6% of second-trimester fetuses and almost always disappear on their own by 23 weeks. Their main association is with trisomy 18, where they appear in 30 to 50% of affected fetuses, but they occur at the same rate in babies with Down syndrome as in babies with no chromosomal differences at all. If you’ve already had low-risk results from earlier screening (like cell-free DNA testing or first-trimester screening), a single isolated soft marker rarely changes the overall picture. Your provider will explain how any soft marker fits into the context of your other results.
When the Baby Won’t Cooperate
Because the sonographer needs very specific angles of very specific structures, baby position matters. If your baby is curled up, facing your spine, or simply not moving into the right position, the sonographer may not be able to complete the checklist in one visit. You might be asked to walk around, drink cold water, or shift positions to encourage movement. If that doesn’t work, you’ll be scheduled for a follow-up appointment to capture the remaining views. This is common and not a sign that anything is wrong.
Finding Out the Sex
If you want to know the sex of your baby, this is usually the appointment where the sonographer can tell you. Let them know at the start whether you’d like that information. If you don’t want to know, say so early so they can avoid accidentally revealing it while walking you through the images. Keep in mind that even at 20 weeks, baby positioning can occasionally make it difficult to get a clear view, so a definitive answer isn’t always possible.
After the Scan
In many practices, the sonographer performs the scan but doesn’t interpret results on the spot. A radiologist or your OB reviews the images afterward, and you’ll discuss the findings at your next prenatal visit or get a call if anything needs attention sooner. Some providers do review results with you the same day, so the experience varies. If the scan is fully normal, it’s typically the last detailed anatomy ultrasound of the pregnancy unless a reason for additional imaging comes up later.