A 20-week anatomy ultrasound is a routine prenatal examination performed around the midpoint of pregnancy. This scan offers a detailed look into the developing fetus, assessing its growth, organ formation, and identifying structural anomalies. Down syndrome is a genetic condition caused by the presence of an extra copy of chromosome 21. While the 20-week ultrasound does not diagnose Down syndrome directly, it plays a role in identifying certain physical characteristics that might suggest an increased likelihood of its presence.
What the 20-Week Ultrasound Examines
During the 20-week ultrasound, the sonographer meticulously examines various parts of the fetus, including the heart, brain, head, neck, and spine. They also inspect the kidneys, bladder, arms, legs, hands, fingers, feet, and toes, along with the lips, chin, nose, eyes, face, chest, lungs, stomach, and intestines. This detailed evaluation helps to ensure that the fetus is growing as expected and that its internal organs are developing properly.
The ultrasound also assesses the umbilical cord, checking its blood flow and attachment to the placenta. The position of the placenta and the amount of amniotic fluid surrounding the fetus are also evaluated. This scan functions as a screening tool, identifying potential concerns and physical characteristics or “markers” that may indicate an increased risk for certain conditions, including Down syndrome.
Specific Ultrasound Markers for Down Syndrome
During the 20-week ultrasound, sonographers look for specific anatomical findings, often referred to as “soft markers,” that may suggest an increased risk of Down syndrome. One such marker is nuchal fold thickening, which refers to an increased amount of skin at the back of the fetal neck, typically measured as 6 millimeters or more between 15 and 20 weeks of gestation. While many babies with a thickened nuchal fold are healthy, it can be a soft marker for chromosomal conditions.
Heart defects are another common finding associated with Down syndrome, with conditions such as atrioventricular septal defect (AVSD) being more prevalent. Duodenal atresia, a blockage in the duodenum, can also be a marker, often presenting as a “double bubble” sign on ultrasound due to an enlarged stomach and duodenal cap. This condition is found in approximately 30% of cases of chromosomal defects, primarily trisomy 21 (Down syndrome).
An absent or hypoplastic nasal bone, meaning the nasal bone is not seen or appears smaller than normal, is another soft marker. Although present in 1-3% of normal babies, it is absent in about 40-60% of babies with chromosomal abnormalities. Echogenic bowel, where the fetal intestines appear unusually bright on the ultrasound, similar to bone, can also be observed. This finding is associated with Down syndrome in about 15% of affected fetuses. Additionally, pyelectasis, a dilation or extra fluid in the renal pelvis of the kidneys, is considered a soft marker.
Understanding Ultrasound Limitations and Accuracy
The 20-week ultrasound is a screening tool, and it cannot definitively diagnose Down syndrome. This means that while it can identify features that raise suspicion, it does not confirm the presence of the condition. Many babies who show one or more ultrasound markers for Down syndrome are born without the condition, which are considered false positives. For example, a thickened nuchal fold or echogenic bowel can occur in healthy pregnancies.
Conversely, some babies with Down syndrome may not display any visible markers on the 20-week ultrasound. These instances are known as false negatives. The absence of markers on the scan does not guarantee that Down syndrome is not present. The accuracy of ultrasound in detecting Down syndrome varies, and its findings are not conclusive on their own. Therefore, ultrasound results are interpreted as an assessment of risk, guiding decisions for further evaluation rather than providing a final diagnosis.
Next Steps Following Ultrasound Findings
If the 20-week ultrasound identifies potential markers for Down syndrome, the next step typically involves consultation with a healthcare provider or a genetic counselor. These discussions help families understand the ultrasound findings, their implications, and the available options for further testing. Additional tests are offered to provide more definitive information.
One such option is Non-Invasive Prenatal Testing (NIPT), a screening blood test that analyzes fragments of fetal DNA circulating in the mother’s bloodstream. NIPT can be performed as early as 10 weeks of pregnancy and is highly reliable for screening for chromosomal disorders like Down syndrome, identifying over 99% of cases.
For a conclusive diagnosis, invasive diagnostic procedures are available. Amniocentesis involves taking a small sample of amniotic fluid, typically between 15 and 20 weeks of pregnancy, which is then analyzed for chromosomal abnormalities. Chorionic Villus Sampling (CVS) is another diagnostic option, performed earlier in pregnancy, usually between 10 and 13 weeks, by taking a tissue sample from the placenta. Both amniocentesis and CVS can definitively diagnose Down syndrome but carry a small risk of miscarriage.