Anatomy scans are a common part of prenatal care, offering expectant parents a detailed look at their developing baby. Many wonder about the scan’s ability to detect conditions like Down syndrome, a genetic disorder caused by an extra chromosome 21. While these ultrasounds provide valuable insights into a baby’s physical structure, their role in identifying genetic conditions is distinct from definitive diagnostic tests.
Understanding the Anatomy Scan
The anatomy scan, often called a Level 2 ultrasound or the 20-week scan, typically occurs between 18 and 22 weeks of pregnancy. This detailed ultrasound comprehensively assesses the developing fetus. Its objective is to evaluate fetal growth and ensure the baby is progressing as expected.
During the scan, various organs and body parts are examined for typical development, including the heart, brain, kidneys, spine, limbs, and facial features. Sonographers also check the placenta’s position and measure the amniotic fluid. The scan aims to confirm the baby’s physical health and identify any structural variations needing further evaluation.
Identifying Potential Indicators
An anatomy scan does not directly diagnose Down syndrome, but it can reveal certain physical characteristics or “soft markers” more frequently observed in fetuses with the condition. These markers are subtle developmental variations that indicate a potential chromosomal abnormality, not a definitive diagnosis.
Common soft markers include an increased nuchal fold thickness, an excess accumulation of fluid or tissue at the back of the baby’s neck. The absence or underdevelopment of the nasal bone is another frequently observed marker. Skeletal abnormalities, such as a shortened femur (thigh bone) or humerus (upper arm bone), can also be identified.
Other potential indicators include echogenic bowel, where the fetal bowel appears unusually bright on the ultrasound, and mild pyelectasis, a slight dilation of the renal pelvis in the kidneys. Serious structural anomalies, particularly heart defects, are also more prevalent in fetuses with Down syndrome and can be detected. The presence of one or more of these markers suggests an elevated risk, prompting further investigation.
Interpreting Scan Findings
Discovering potential indicators during an anatomy scan does not automatically confirm a diagnosis of Down syndrome. Many soft markers can also be present in typically developing fetuses without chromosomal abnormalities. These findings are “soft” markers because they are not specific to Down syndrome and can lead to false positives.
A false positive occurs when a marker is identified, but the baby does not have Down syndrome. Conversely, a false negative can happen when no markers are observed, yet the baby is born with the condition. The anatomy scan functions as a screening tool, identifying pregnancies that might benefit from additional evaluation rather than providing a conclusive diagnosis.
The significance of any identified markers is often weighed against other factors, such as maternal age and results from other prenatal screenings. If soft markers are detected, healthcare providers typically discuss the findings, their implications, and available options for further assessment with expectant parents.
Diagnostic Confirmation of Down Syndrome
For a definitive diagnosis of Down syndrome, diagnostic tests are necessary, as the anatomy scan only provides indicators. Non-invasive prenatal testing (NIPT) is a highly accurate screening test performed as early as 10 weeks. NIPT analyzes fetal DNA fragments in the mother’s blood to assess the risk of chromosomal conditions, including Down syndrome. While reliable for screening, a high-risk result still requires confirmation.
The two primary diagnostic procedures are chorionic villus sampling (CVS) and amniocentesis. CVS is typically performed between 10 and 13 weeks, involving a small tissue sample from the placenta. Amniocentesis is usually done between 15 and 20 weeks, collecting amniotic fluid. Both procedures obtain fetal cells for chromosomal analysis, providing a definitive diagnosis.
These invasive diagnostic tests carry a small risk of complications, such as miscarriage, generally less than 0.5% for amniocentesis and slightly higher for CVS. The decision to proceed with these tests is personal, made after considering potential risks and benefits, often with genetic counselors. These tests offer clarity where screening methods only provide probabilities.