The 20-week anatomy scan, often called the anomaly scan, is a significant milestone that brings a mix of excitement and anxiety for expectant parents. This detailed ultrasound offers the first comprehensive look at the developing fetus, leading to concern about potential unexpected findings. Understanding the procedure’s goals and the statistical reality of its findings provides an evidence-based perspective.
The Primary Goal of the Anomaly Scan
The fundamental purpose of the 20-week scan is a systematic, non-invasive screening of the fetal structure and environment, not to provide a definitive diagnosis. It is performed between 18 and 22 weeks of gestation because the fetal organs are sufficiently developed to be visualized clearly. The sonographer checks the fetus’s size and growth progression against established norms.
This systematic examination includes a detailed review of the major organ systems and structures. Specific attention is given to the four chambers of the heart and the great vessels, the symmetry and structure of the brain, and the integrity of the spine. The scan also checks the kidneys, bladder, stomach, limbs, and facial profile for structural irregularities. Beyond the fetus, the assessment involves checking the location of the placenta and ensuring the amniotic fluid volume is within a healthy range.
Understanding the Statistical Likelihood of Major Findings
The anxiety surrounding this appointment often far outweighs the statistical probability of a major finding. For the vast majority of pregnancies, the scan provides significant reassurance, confirming that the fetus is developing normally. Studies consistently show that 95% to 98% of all 20-week anatomy scans identify no significant structural anomalies.
Only a small percentage of pregnancies, typically between 2% and 5%, will have any finding at all, including both minor and major issues. The percentage of scans revealing a major structural anomaly that requires specialized care or intervention is even smaller. Congenital heart defects are among the most common major findings, occurring in about 1 in 100 pregnancies, but many other anomalies are even less frequent.
Distinguishing Between Significant Anomalies and Soft Markers
When an unexpected result occurs, it is important to distinguish between a significant structural anomaly and a “soft marker.” A significant anomaly is a clear structural defect that will likely require medical attention or intervention after birth, such as spina bifida, severe congenital heart disease, or bilateral renal agenesis. The detection of a significant anomaly necessitates immediate consultation with specialists to plan for specialized delivery and postnatal care.
A soft marker is a minor sonographic variation or finding that is not a structural defect itself but is statistically associated with a slightly increased risk for a chromosomal condition, such as Down syndrome. Examples include an Echogenic Intracardiac Focus (EIF) or mild kidney dilation, known as pyelectasis. These markers are often transient, resolve before birth, or are benign normal variations found in healthy fetuses. When a soft marker is found in isolation, especially when previous screenings like Non-Invasive Prenatal Testing (NIPT) were low-risk, the increase in risk for a chromosomal condition is generally quite small.
What Happens Following an Unexpected Result
If the sonographer identifies a finding, the next steps focus on definitive confirmation and consultation. The initial finding from the screening ultrasound is not considered a final diagnosis. The first step involves a referral for a more specialized and detailed ultrasound, often called a Level II scan, typically performed by a Maternal-Fetal Medicine (MFM) specialist.
The MFM specialist confirms the initial finding and assesses its severity, often using advanced imaging like a fetal echocardiogram for heart-related concerns. Following the imaging, a consultation with a genetic counselor is common, especially if a soft marker is present or a chromosomal condition is suspected. The counselor helps interpret the data and explains the option of diagnostic tests, such as an amniocentesis, which can provide a definitive answer about chromosomal abnormalities.