Down syndrome, also known as Trisomy 21, is a genetic condition resulting from an extra copy of chromosome 21. This extra genetic material influences development. Prenatal ultrasound is a non-invasive medical imaging technique used during pregnancy, using sound waves to create real-time images of the fetus. Ultrasounds monitor fetal growth, development, and overall health.
Key Ultrasound Markers for Down Syndrome
During a prenatal ultrasound, healthcare providers look for specific physical characteristics, known as soft markers or anatomical anomalies, suggesting an increased likelihood of Down syndrome. One marker is increased nuchal translucency (NT), an accumulation of fluid at the back of the fetal neck. Another common finding is an absent or underdeveloped nasal bone. These features are assessed for their presence and development.
Another indicator is echogenic bowel, where fetal intestines appear unusually bright, similar to bone. Shortened long bones, particularly the femur (thigh bone) and humerus (upper arm bone), are also measured. Mild dilation of the fetal kidney, known as pyelectasis, is sometimes detected. These findings provide structural details about the fetus.
Heart defects are common in fetuses with Down syndrome, including ventricular septal defects (VSDs) or atrioventricular septal defects. Ultrasound can visualize fetal heart structure to identify anomalies. Less common markers include a “sandal gap” (increased space between the first and second toes) and clinodactyly (an incurved little finger). These markers offer clues during prenatal assessment.
When Ultrasound Markers Are Observed
Ultrasound timing is important for identifying specific Down syndrome markers. First-trimester screening, performed between 11 and 14 weeks, includes the nuchal translucency scan. This scan measures the fluid behind the fetal neck and assesses the nasal bone. Observing these markers provides early indications.
The second-trimester anatomy scan, between 18 and 22 weeks, is a more comprehensive examination. During this scan, a wider range of fetal structures and organs are assessed, detecting other markers like heart defects, echogenic bowel, or shortened long bones. Markers become more reliably visible or measurable at these stages of development.
Understanding Ultrasound’s Role in Screening
Ultrasound is a screening tool, not a definitive diagnostic test for Down syndrome. The presence of one or more ultrasound markers increases the estimated risk of the condition but does not confirm a diagnosis. Many markers, when found in isolation, can also be present in pregnancies without Down syndrome. For example, a shortened femur can be a normal variant in a healthy baby.
This highlights false positives and false negatives in screening. A false positive occurs when markers suggest increased risk, but the baby is born without Down syndrome. Conversely, a false negative occurs when no markers are seen, yet the baby is later diagnosed. Ultrasound provides an assessment of risk, not a conclusive answer.
Following Up on Ultrasound Findings
When ultrasound findings suggest increased Down syndrome risk, several options for further evaluation are available. Non-invasive prenatal testing (NIPT) is often offered. This blood test analyzes fragments of fetal DNA circulating in the mother’s bloodstream to screen for chromosomal abnormalities, providing a more accurate risk assessment than ultrasound.
For a definitive diagnosis, invasive procedures like chorionic villus sampling (CVS) or amniocentesis are recommended. CVS is performed earlier, between 10 and 13 weeks, by taking a placental tissue sample. Amniocentesis, done after 15 weeks, involves collecting amniotic fluid. Both provide genetic material for chromosomal analysis, offering a conclusive diagnosis.
Genetic counseling offers families detailed information about ultrasound findings, testing options, and potential outcomes. Counselors discuss test benefits, risks, and accuracy, helping parents make informed decisions. This support ensures families understand their choices for prenatal management.