KEY FACTS
Terminology
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Autosomal trisomy of chromosome 18 (T18)
Imaging
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Hallmark finding: Multiple anomalies
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No single anomaly is hallmark of T18
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Most cases with findings at time of nuchal translucency (NT)
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↑ NT, absent nasal bone, anomalies
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2nd-trimester anomalies in almost all fetuses
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Cardiac anomaly, omphalocele (often bowel only), diaphragmatic hernia, spina bifida, brain anomalies
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Musculoskeletal anomalies
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Radial ray, contractures (arthrogryposis), rocker-bottom feet (convex sole of foot)
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Clenched hands + overlapping index finger
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Markers are rarely isolated
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Choroid plexus cysts (CPC), single umbilical artery, strawberry-shaped calvarium
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Fetal growth restriction: Early, progressive, and severe
Top Differential Diagnoses
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Trisomy 13: Holoprosencephaly is hallmark anomaly
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Triploidy: FGR and cystic placenta is hallmark
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Pena-Shokeir syndrome: Fetal akinesia/hypokinesia
Clinical Issues
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Associated with advanced maternal age
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Cell-free fetal DNA detection rate is 96%
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Maternal serum quadruple test with 80% detection
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1/2 with intrauterine fetal demise (IUFD)
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Ultrasound is poor predictor for IUFD
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20% survive beyond 1 month, 5-10% beyond 1st year
Scanning Tips
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Obtain additional cardiac and extremity views if markers seen at time of anatomy scan
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CPC will resolve regardless if fetus has T18 or not
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Structural anomalies at time of NT seen best with transvaginal ultrasound
and ventriculomegaly 
are seen.
, abnormal heart axis 
, and small left ventricle 
are also seen on the 4-chamber cardiac view. Not a single hallmark anomaly is associated with trisomy 18; however, the diagnosis should be suspected with multiple anomalies in different organ systems.
. Multiple other anomalies were also present.
