The anatomy scan, often called the Level II ultrasound or 20-week scan, is a routine medical appointment scheduled between 18 and 22 weeks of pregnancy. This detailed examination is designed to assess the developing fetus, checking growth measurements and structural anatomy to ensure all systems are progressing as expected. A common question for expectant parents is whether this procedure is performed internally; the standard and primary method for the anatomy scan is transabdominal, meaning the imaging is done externally over the abdomen.
The Standard Approach: Transabdominal Ultrasound
The majority of the anatomy scan uses a transabdominal technique, where a transducer wand is placed directly on the pregnant person’s skin. A warm, water-based gel is applied to the lower abdomen to ensure a seamless connection, allowing high-frequency sound waves to travel without disruption. These waves reflect off internal structures, and the transducer captures the echoes to create two-dimensional images on a screen.
By the mid-second trimester, the uterus has grown significantly, making the transabdominal approach effective for visualization. The fetus is large enough for the sonographer to capture fine anatomical details, and sufficient amniotic fluid enhances image clarity. The sonographer moves the transducer across the abdomen, sometimes applying pressure, to obtain necessary views and measurements. This external method is generally comfortable, though the scan is often the longest part of the appointment, sometimes lasting between 45 and 75 minutes depending on the fetus’s position.
When a Transvaginal View is Necessary
While the primary method is transabdominal, a transvaginal (internal) ultrasound may be requested immediately following the external check for specific clinical reasons. This internal method involves inserting a smaller, lubricated transducer a short distance into the vagina to get closer to the structures in the lower pelvic area. The higher frequency of the transvaginal probe allows for clearer, higher-resolution images of structures that are near the cervix.
A frequent reason for this supplemental scan is to accurately measure the length of the cervix, which is a key predictor of the risk for preterm labor. The transvaginal view provides a more precise measurement than the external method, particularly when the fetus is lying low in the pelvis. Additionally, it may be used if the sonographer needs a better look at the placenta’s position, especially if concerns about a low-lying placenta or placenta previa arise from the abdominal scan.
Preparing for and Experiencing the Anatomy Scan
Preparation for the anatomy scan is focused on maximizing image clarity and ensuring patient comfort during the procedure. While early pregnancy ultrasounds often require a full bladder, requirements for the 20-week scan can vary between clinics. Some facilities may request a full or partially full bladder for the transabdominal portion, while others prefer an empty bladder to better visualize the fetal anatomy.
The total appointment time is usually scheduled for at least an hour, allowing the sonographer sufficient time to capture required images and measurements. During the scan, the sonographer focuses on technical requirements and may need the pregnant person to shift positions to encourage the fetus to move for better views. It is helpful to wear comfortable clothing, such as a two-piece outfit, that allows the abdomen to be easily exposed for the application of the gel.
Detailed Checklist of Fetal Anatomy
The anatomy scan provides a detailed structural survey of the fetus, moving beyond simple dating and growth assessment. The sonographer systematically examines the fetal brain, looking at the cerebellum, ventricles, and other structures to confirm normal development and symmetry. A primary focus is the fetal heart, where the four chambers, valves, and major blood vessels are assessed to rule out structural cardiac anomalies.
The spine is examined along its entire length, both in cross-section and longitudinally, ensuring the vertebrae are correctly aligned and the skin is fully closed over the spinal cord. Other organs are also checked, including the stomach, which should contain fluid, confirming the fetus is swallowing amniotic fluid. The scan confirms the presence of two kidneys and the filling and emptying of the fetal bladder, which demonstrates proper kidney function. Finally, the sonographer counts the vessels in the umbilical cord, assesses the location of the placenta, and measures the volume of amniotic fluid surrounding the fetus.