The 20-week anatomy scan, sometimes referred to as the anomaly scan or mid-pregnancy ultrasound, is an examination typically performed between the 18th and 22nd weeks of pregnancy. This procedure uses sound waves to create images of the developing fetus, providing the first comprehensive look at its physical structure. The primary objective is to systematically assess the baby’s development, confirm growth is on track, and screen for structural variations or congenital conditions that might require monitoring or specialized care. This appointment serves as a significant milestone, offering reassurance about the progress of the pregnancy and gathering detailed data for the healthcare team.
Preparing for the Scan and the Procedure
Before the appointment, you may receive specific instructions regarding your bladder, which can vary by clinic; some may request a partially full bladder to improve image clarity, while others prefer an empty bladder for better fetal visualization. Wearing comfortable, two-piece clothing is recommended, as you will need to expose your abdomen for the sonographer to apply the ultrasound gel and move the transducer. The scan is performed in a dimly lit room to enhance image visibility and generally takes between 30 and 60 minutes, depending on the baby’s position and cooperation.
Once you are lying down, the sonographer will apply gel to your belly, which helps the transducer make effective contact and transmit the sound waves. The transducer is then gently pressed against your skin and moved across the abdomen to capture various cross-sectional images of the fetus and surrounding structures. The sonographer will often focus silently to concentrate on obtaining the required measurements and views, sometimes pausing to calculate dimensions on the screen.
While the medical assessment is the central purpose of the visit, the 20-week scan is often the first opportunity to determine the sex of the fetus, provided the baby is positioned favorably. You should communicate your preference regarding learning the sex to the sonographer at the start of the appointment. Occasionally, if the baby is resting in a difficult position, you may be asked to walk around or drink something cold to encourage movement, helping the technician capture all necessary anatomical views.
Detailed Anatomical Survey and Measurements
The sonographer performs a methodical, head-to-toe evaluation, starting with standard growth metrics known as fetal biometry. These measurements include the biparietal diameter (BPD), the distance between the two sides of the head, and the head circumference (HC), which assesses the overall size of the skull. The abdominal circumference (AC) is measured to gauge overall body mass and growth, and the femur length (FL) tracks long bone development.
A comprehensive assessment of the internal organs checks for normal formation and function, with particular attention paid to the fetal heart. The sonographer obtains a four-chamber view of the heart to ensure the two upper atria and two lower ventricles are present and correctly sized, along with checking the flow through the major vessels. In the abdomen, the presence of a stomach bubble confirms the baby is swallowing amniotic fluid, and visualizing two kidneys and a filled bladder indicates the urinary system is functioning.
The spine is examined along its entire length to ensure the vertebrae are properly aligned and the skin is closed over the spinal cord. The face is visualized to look for signs of a cleft lip or palate, and the limbs are checked to confirm the appropriate number of bones, hands, and feet. The sonographer also assesses brain structures, including the cerebellum and the lateral ventricles; a width of 10 millimeters or greater in the lateral ventricle atrium requires further attention.
Beyond the fetus, the scan evaluates the structures supporting the pregnancy within the uterus. The location of the placenta is documented relative to the cervix, which is important for delivery planning. The umbilical cord is checked to ensure it contains the expected three vessels—two arteries and one vein—and the volume of amniotic fluid surrounding the baby is measured. The amount of amniotic fluid is an indicator of fetal well-being, as levels that are too low or too high can suggest an underlying issue.
Understanding the Results and Follow-Up
Following the examination, the images and measurements are typically reviewed by a physician, such as a maternal-fetal medicine specialist, who prepares a final report. The sonographer may provide a brief summary, but the results are usually discussed with your primary healthcare provider at a follow-up appointment. The majority of 20-week scans yield reassuring results, confirming normal growth and development.
If the scan reveals a finding, it may fall into two broad categories: a soft marker or a major structural anomaly. Soft markers are subtle findings, such as a choroid plexus cyst in the brain, which are usually minor and often resolve on their own, but can sometimes be associated with an increased risk for a chromosomal condition. A more significant finding, like a suspected heart defect or a discrepancy in fetal growth, is considered a major anomaly and triggers an immediate action plan.
Should any concern be identified, the next steps involve further investigation and consultation with specialized medical professionals. This may include a referral to a maternal-fetal medicine specialist for a Level 2 or targeted ultrasound, which provides higher resolution of the area of concern. Other follow-up testing might involve a fetal echocardiogram for a detailed assessment of the heart or, in rare cases, an amniocentesis to test for genetic or chromosomal abnormalities. These steps guide ongoing care and prepare for any necessary interventions after birth.