The anatomy scan, also known as the detailed or Level II ultrasound, is a routine, non-invasive imaging procedure performed during the second trimester of pregnancy. This comprehensive evaluation serves as a major milestone, offering the first thorough look at the developing fetus. Understanding the objectives of the scan and the reasons for its scheduling helps to demystify this important part of prenatal care.
What the Anatomy Scan Evaluates
The primary goal of the anatomy scan is to perform a detailed assessment of the fetal structure and overall well-being. The sonographer systematically examines the fetus from head to toe, taking numerous measurements that are compared against expected growth parameters for the gestational age.
Biometry and Fluid Assessment
Standard measurements, including the circumference of the head and abdomen, along with the length of the femur, are taken to estimate fetal weight and confirm the growth rate. The technician also assesses the placenta’s position within the uterus, specifically checking for a low-lying location near the cervix. Additionally, the volume of amniotic fluid surrounding the fetus is evaluated, as this fluid is necessary for proper lung development and fetal movement.
Organ System Review
The scan involves a meticulous check of all major organ systems.
- The brain is examined for normal development of key structures, such as the cerebellum and the fluid-filled ventricles.
- The spine is viewed in both longitudinal and cross-section planes to ensure the vertebrae are properly aligned and covered by skin.
- The fetal heart is assessed, requiring visualization of all four chambers and the proper outflow tracts for blood circulation.
- The kidneys, stomach, and bladder are examined to confirm their presence and function.
- The sonographer inspects the limbs, counting fingers and toes, and checking the integrity and position of the arms and legs.
The Optimal Timing Window for Fetal Assessment
Medical consensus places the ideal window for the detailed anatomy scan between 18 and 22 weeks of gestational age. This timeframe is strategically chosen to balance the need for fetal development with the quality of the ultrasound image. By 18 weeks, the fetus has generally developed enough major structures to allow for a meaningful evaluation. The period around 20 weeks is often cited as the gold standard because the fetus is large enough for detailed visualization, and the surrounding amniotic fluid is typically at an adequate volume. Sufficient amniotic fluid helps to create a good acoustic window, which is necessary for the sound waves to produce clear images.
Specific Considerations for an 18-Week Scan
Eighteen weeks is recognized as the very beginning of the optimal window, meaning a scan at this time is generally considered acceptable. It is not too early from a medical standpoint, but it does sit at the threshold where technical limitations are more common. The main advantage of scanning at 18 weeks is the earlier detection of any major structural issues, which can provide peace of mind or allow more time for parental planning and specialized consultation.
The primary trade-off is the increased likelihood of an incomplete examination compared to a scan performed at 20 or 21 weeks. Studies indicate that a scan completed between 18.0 and 18.9 weeks gestation is approximately twice as likely to be incomplete as one performed at 19.0 weeks or later. This is due to the smaller size of the fetus at 18 weeks, making smaller structures, such as the four chambers of the heart or the extremities, more challenging to visualize clearly. If the sonographer cannot obtain adequate views, a follow-up scan, often called a “rescan,” will be recommended two to four weeks later.
When Earlier Screening May Be Recommended
In some clinical scenarios, a physician may intentionally schedule an anatomy survey at 18 weeks or even a targeted, limited anatomy scan earlier than that. This deviation from the 20-week average is driven by specific maternal or fetal risk factors, allowing for proactive monitoring and intervention planning. Patients with a history of congenital anomalies or a family history of certain genetic conditions may require a specialized, earlier assessment. Maternal conditions like pre-existing diabetes or significant obesity can also necessitate an earlier scan due to the increased risk of certain fetal anomalies. For pregnancies involving twins or other multiple gestations, earlier and more frequent monitoring is often required, where the benefit of early information outweighs the potential for a technically incomplete initial study.