The mid-pregnancy ultrasound, commonly known as the anatomy scan, is a routine procedure typically performed between 18 and 22 weeks of gestation. This comprehensive imaging test is a screening tool designed to evaluate the physical development of the fetus, not a diagnostic test for genetic conditions. While the scan cannot definitively diagnose Down Syndrome (Trisomy 21), it can identify physical findings that may increase the statistical likelihood of the condition, prompting further discussion about specific genetic testing.
The Purpose and Scope of the Anatomy Scan
The primary goal of the anatomy scan is to conduct a detailed, systematic survey of the developing fetus’s anatomy. Sonographers carefully examine the major organs and structures to detect any significant structural anomalies or malformations, including the skull, brain, spine, heart, lungs, kidneys, and limbs.
The scan also assesses the overall health and progress of the pregnancy. Measurements are taken to estimate fetal growth and weight, confirming appropriate development for gestational age. The technician also evaluates the location of the placenta, the volume of amniotic fluid, and the integrity of the umbilical cord.
Evaluating the four chambers of the heart and major blood vessels is a substantial part of the examination, as congenital heart defects are common anomalies. This information establishes a baseline of fetal health and is crucial for anticipating potential complications or planning specialized care after birth.
Physical Indicators of Down Syndrome (Soft Markers)
Specific sonographic features identified during the anatomy scan are statistically associated with an increased chance of Trisomy 21. These findings are categorized as “soft markers” because they are minor variations that can also appear in genetically typical fetuses. The presence of these markers alone is not a diagnosis, but indicates that further screening or diagnostic evaluation may be warranted.
One significant soft marker is increased nuchal fold thickness, where the skin at the back of the neck measures 6 millimeters or more. Other common soft markers include an echogenic intracardiac focus (EIF), which appears as a bright spot in the heart, and hyperechogenic bowel, where the fetal bowel appears unusually bright. Shortening of the long bones, such as the femur or humerus, is also a marker, though it is often a benign finding.
The sonographer also looks for major structural anomalies, referred to as “hard markers,” which have a stronger association with genetic conditions. These include major congenital heart defects, such as atrioventricular canal defects, and gastrointestinal issues like duodenal atresia. The risk of Down Syndrome increases with the number of soft markers present; three or more markers can significantly raise the likelihood of the condition.
Limitations of Ultrasound and Diagnostic Certainty
An ultrasound is a screening tool, meaning it can only estimate the risk of a condition, not confirm its presence. The Positive Predictive Value (PPV) of soft markers is low, especially when found in isolation within a low-risk pregnancy. For instance, an echogenic intracardiac focus is frequently observed in genetically typical fetuses and often has no clinical consequence.
The risk estimate from the anatomy scan must be interpreted alongside prior genetic screening results, such as Non-Invasive Prenatal Testing (NIPT). NIPT analyzes cell-free fetal DNA from the mother’s blood, offering a high detection rate for Trisomy 21. A normal anatomy scan can substantially reduce the estimated risk of Down Syndrome, even for women considered high-risk based on age alone.
When multiple soft markers or a hard marker are identified, definitive diagnostic testing is offered to confirm the presence of an extra copy of chromosome 21. Invasive procedures, such as amniocentesis and Chorionic Villus Sampling (CVS), provide a direct analysis of the fetal chromosomes. Amniocentesis is typically performed after 15 weeks, while CVS is done earlier; both carry a small risk of pregnancy loss. These tests provide a near 100% accurate diagnosis through karyotyping, which counts and examines the chromosomes.