Down syndrome, also known as Trisomy 21, is a chromosomal condition caused by an extra copy of chromosome 21. This genetic alteration can influence a baby’s physical development and cognitive abilities. While ultrasound is a valuable prenatal screening tool, it identifies potential indicators of Down syndrome, not a definitive diagnosis. It assesses fetal development and can suggest an increased likelihood of the condition.
Ultrasound as a Screening Tool
Ultrasound plays a significant role in prenatal care by assessing fetal growth and identifying potential anomalies. It’s important to distinguish between screening and diagnosis: screening tests, like ultrasound, estimate risk, while diagnostic tests confirm a condition’s presence or absence. Early in pregnancy, a nuchal translucency (NT) scan is typically performed between 11 and 14 weeks of gestation. This scan measures the fluid-filled space at the back of the baby’s neck.
A second-trimester anatomy scan, usually conducted around 18 to 22 weeks, offers a more detailed examination of the developing fetus. This scan evaluates the baby’s organs and structures, looking for any physical signs that might be associated with chromosomal conditions. These ultrasound examinations are often integrated with other prenatal screening methods, such as maternal blood tests, to provide a more comprehensive risk assessment for Down syndrome.
Common Ultrasound Markers
During an ultrasound, healthcare providers look for specific “soft markers” or anatomical features more commonly observed in fetuses with Down syndrome. An increased nuchal translucency measurement, typically above 3.5 mm in the first trimester, is one such marker. Another potential indicator is an absent or hypoplastic nasal bone, meaning it appears smaller than expected or is not visible. This finding is more prevalent in fetuses with Down syndrome.
Other soft markers identified during the second-trimester anatomy scan include an echogenic bowel (fetal intestines appear unusually bright) and an echogenic intracardiac focus (EIF), a small bright spot in the baby’s heart.
Shortened long bones (femur or humerus), mild pyelectasis (slight enlargement of the fetal kidney’s collecting system), and choroid plexus cysts (small fluid-filled areas in the brain) can also be observed. These markers are minor findings and do not necessarily indicate a problem.
Understanding Ultrasound Results and Limitations
The presence of ultrasound markers increases the estimated likelihood of Down syndrome but does not confirm a diagnosis. Conversely, their absence does not guarantee a baby does not have the condition. Ultrasound findings can lead to false positives (markers present, no Down syndrome) or false negatives (no markers, condition present).
To provide a more accurate risk assessment, ultrasound findings are typically combined with other screening methods. Non-invasive prenatal testing (NIPT), a blood test that analyzes fetal DNA in the mother’s bloodstream, is often used in conjunction with ultrasound. This integrated approach helps refine the risk estimation, guiding decisions about whether further diagnostic testing is warranted.
Next Steps for Definitive Diagnosis
When ultrasound findings, combined with other screening results, suggest a heightened risk of Down syndrome, diagnostic options become available. These invasive procedures analyze the baby’s chromosomes to provide a definitive diagnosis.
Chorionic villus sampling (CVS) involves taking a placental tissue sample (10-13 weeks). Amniocentesis withdraws amniotic fluid (15-20 weeks). Both analyze fetal cells for the extra chromosome 21, confirming or ruling out Down syndrome with high accuracy. These procedures carry a small risk of complications, including miscarriage. Genetic counseling is important to discuss options, implications, and risks, ensuring informed decision-making.