KEY FACTS
Imaging
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1st-trimester findings (11- to 14-week scan)
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↑ nuchal translucency: ↑ fluid behind neck on midsagittal view
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Absent nasal bone
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Abnormal ductus venosus and tricuspid flow
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2nd-trimester minor markers (15-22 weeks)
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↑ nuchal fold thickness (≥ 6 mm)
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Absent or small nasal bone
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Short femur length/short humerus length
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Echogenic bowel
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Intracardiac echogenic focus
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Renal pelvis dilatation
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Mild lateral ventriculomegaly
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Hallmark anomalies associated with trisomy 21
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Atrioventricular septal defect
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Esophageal atresia (EA)
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Duodenal atresia (DA)
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Top Differential Diagnoses
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Turner syndrome: Cystic hygroma is hallmark anomaly
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Trisomy 18: Multiple anomalies
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Trisomy 13: Holoprosencephaly is hallmark anomaly
Diagnostic Checklist
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Associated with advanced maternal age ( ≥ 35 years old)
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Cell-free fetal DNA (cfDNA) is best screening test
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cfDNA detects 99.2% (0.1% false-positive rate)
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Maternal serum markers detect 95% (3-5% false + rate)
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Chorionic villus sampling and amniocentesis are true diagnostic tests
Scanning Tips
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Hepatomegaly/splenomegaly in 3rd trimester suggestive of fetal transient abnormal myelopoiesis or leukemia
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Polyhydramnios from EA or DA often in 3rd trimester
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Look at profile carefully (big tongue, flat midface)
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Ultrasound may be completely normal (40-50%)
. Maternal serum screen results were suspicious for trisomy 21 (T21) and amniocentesis in the 2nd trimester was performed, diagnostic for T21. The pregnancy was continued.
(≥ 4 mm in midgestation) is seen in this 2nd-trimester fetus with T21. This finding is considered both a marker for aneuploidy and potential progressive hydronephrosis.
and mitral and tricuspid valves positioned at the same level 
, typical findings of atrioventricular septal defect (AVSD). Up to 1/2 of the fetuses with AVSD have aneuploidy; most common diagnosis is T21.
