Down syndrome, also called trisomy 21, is the most common chromosomal abnormality in live-born infants, caused by the presence of an extra copy of chromosome 21. Prenatal ultrasound plays a key role in screening for Down syndrome through several markers. In the first trimester, increased nuchal translucency, a measurement of fluid at the back of the fetal neck, is the primary ultrasound screening tool. In the second trimester, sonographic markers include a thickened nuchal fold, absent or small nasal bone, shortened femur and humerus, echogenic bowel, echogenic intracardiac focus, and certain heart defects. While ultrasound markers alone cannot diagnose Down syndrome, they help identify pregnancies at higher risk that may benefit from definitive genetic testing.
Sonography Term