The 20-week ultrasound is an external scan. A sonographer applies gel to your abdomen and moves a handheld device called a transducer across your belly to capture images of the baby. In rare cases, an internal (transvaginal) probe may also be used, but the standard anatomy scan is performed entirely on the outside.
What Happens During the Scan
You’ll be asked to slightly lower your pants and lift your shirt. The sonographer spreads a layer of warm gel on your abdomen, then presses the transducer against different spots to get clear views of the baby from multiple angles. The gel helps sound waves travel between the device and your skin, producing a real-time image on the monitor beside you.
The scan is painless, though the pressure of the transducer can feel mildly uncomfortable when pressed over a full bladder. Some clinics ask you to drink water beforehand so your bladder is full, which pushes the uterus into a better position for imaging. Others don’t require this, so check your appointment instructions ahead of time. Wearing loose clothing that’s easy to pull up from the waist saves time and hassle.
When an Internal Probe Is Added
A transvaginal ultrasound uses a slim, wand-shaped probe inserted into the vagina. At 20 weeks, this is not the default approach, but your provider may add it in a few specific situations:
- Cervical length measurement. If you have risk factors for preterm labor or cervical insufficiency, a transvaginal probe gives a much more accurate measurement of your cervix than an abdominal view can.
- Low-lying placenta. When the placenta appears close to or covering the cervix on the abdominal scan, a transvaginal view helps confirm the exact position.
- Limited visibility. A higher BMI or low amniotic fluid levels can make it harder to see certain structures through the abdomen. The internal probe sits closer to the uterus and can fill in the gaps.
If a transvaginal scan is needed, the sonographer will explain why before proceeding. It typically adds only a few minutes to the appointment.
What the Scan Checks
The 20-week ultrasound is often called the anatomy scan because its main purpose is a head-to-toe survey of the baby’s developing body. The sonographer works through a detailed checklist that covers nearly every organ system visible on ultrasound.
For the head, they check the skull shape and integrity, measure head circumference, and examine the brain structures including the cerebral ventricles, cerebellum, and surrounding fluid spaces. They look at the baby’s face in profile, check the eye sockets, and examine the upper lip for signs of a cleft. Moving down, the spine is scanned both lengthwise and in cross-section to confirm each vertebra is forming correctly.
The heart gets particular attention. The sonographer confirms a normal heart rate and rhythm, captures a four-chamber view of the heart, and images the major vessels leaving it. They also examine the lungs, diaphragm, and chest shape. In the abdomen, they look at the stomach, liver, kidneys, bladder, and the point where the umbilical cord inserts into the belly. All four limbs are checked, including the long bones of the arms and legs, hands, and feet, with measurements of the thigh bone used to help estimate gestational age.
The placenta’s position and the volume of amniotic fluid are also assessed. Many clinics now include fetal sex as part of the routine checklist, though some facilities let you choose whether you want to be told.
How Accurate the Scan Is
No ultrasound catches everything. A Cochrane review of screening accuracy found that when both a first-trimester and a second-trimester scan are performed together, about 84% of structural anomalies are correctly identified. When only a single mid-pregnancy scan is done without an earlier screening, detection drops to roughly 50%. For the most severe, life-threatening anomalies, detection rates are considerably higher, around 91%.
Certain conditions are easier to spot than others. Major heart defects and brain abnormalities tend to be detected more reliably than minor limb or facial differences. Baby’s position during the scan matters too. If the baby is curled up or facing your spine, the sonographer may not get clear views of the face or heart on the first attempt. You might be asked to walk around, drink something cold, or come back for a follow-up scan.
Timing and What to Expect
Guidelines recommend scheduling the anatomy scan between 19 weeks 0 days and 21 weeks 0 days for the best balance of visibility and timing. At this stage, the baby is large enough for detailed imaging but still has enough room to move, making it easier to capture all the required views. Earlier scans risk missing structures that haven’t fully developed yet, while later scans can run into issues with the baby’s size and positioning.
Plan for the appointment to take anywhere from 20 to 45 minutes, depending on the baby’s cooperation and whether any additional views are needed. The sonographer typically captures all the clinical images first, then may offer you a chance to see the baby’s face or watch movement on screen. Your provider or a specialist will review the images afterward and contact you with results, sometimes the same day, sometimes within a week.