TRISOMY 21 SOFT MARKERS ULTRASOUND IN 2ND TRIMESTER

Second trimester

Approximately 30% of babies with Down syndrome have detectable abnormalities on the mid-trimester ultrasound ¹.  

Soft markers

Soft markers are sonographic findings that do not in themselves cause any adverse outcomes. However, they are seen more frequently in fetuses with an abnormality.  This article addresses the soft markers that are specific to Down syndrome. For a general discussion, please refer to the article on soft markers.  

The markers are as follows ¹³:

• echogenic intracardiac focus
  • positive LR: 5.83
  • negative LR: 0.8
  • isolated LR: 0.95
• ventriculomegaly
  • positive LR: 27.5
  • negative LR: 0.94
  • isolated LR: 3.8
• nuchal fold thickness >6 mm
  • positive LR: 23.3
  • negative LR: 0.8
  • isolated LR: 3.79
• echogenic bowel
  • positive LR: 11.44
  • negative LR: 0.9
  • isolated LR: 1.65
• hypoplastic/absent nasal bone
  • positive LR: 23.3
  • negative LR: 0.46
  • isolated LR: 6.58
• shortened humerus
  • positive LR: 4.8
  • negative LR: 0.7
  • isolated LR: 0.78
• mild pyelectasis
  • positive LR: 7.6
  • negative LR: 0.9
  • isolated LR: 1.08
• shortened femur
  • positive LR: 3.72
  • negative LR: 0.8
  • isolated LR: 0.61
• aberrant right subclavian artery (ARSA)
  • positive LR: 21.48
  • negative LR: 0.7
  • isolated LR: 3.9

In the presence of soft markers, the risk of Down syndrome is recalculated as new risk = baseline risk x likelihood ratio (LR). The new LR is calculated by multiplying all positive LRs (of markers present) and all negative LRs (of markers absent).

Note: if a single marker is present, then isolated LR is considered.

Structural abnormalities

The following may be present in association with Down syndrome:

• cardiac defects
  • tricuspid regurgitation
  • atrial septal defect (ASD)
  • ventricular septal defect (VSD)
  • atrioventricular septal defect (AVSD)
  • tetralogy of Fallot (less common ¹)
• abdominal
  • duodenal atresia
  • esophageal atresia
  • omphalocele (more common with trisomy 18 ⁹)
• central nervous system
  • mild fetal ventriculomegaly
• craniofacial/calvarial
  • short maxilla
  • mild brachycephaly
• other
  • abnormal ductus venosus waveforms ⁶
  • cystic hygroma
  • hydrops fetalis

Adjunct features

Other features that may be present, but are neither a structural abnormality or a validated soft marker ⁸

• hypoplastic 5^th digit
• wide iliac angle
• shortened frontothalamic distance
• short fetal ear length
• brachydactyly