Echogenic fetal bowel

Echogenic fetal bowel is an observation in antenatal ultrasound imaging, in which fetal bowel appears to be brighter than it is supposed to be. It is a soft marker for trisomy 21 and has several other associations. When observed, it needs to be interpreted in the context of other associated abnormalities.

Epidemiology

The estimated incidence is reported to range from 0.2-1.8% of second trimester fetuses. 

Pathology

The cause of echogenic fetal bowel is uncertain, although it has been postulated that it is related to loss of water from meconium. Note that meconium can become echogenic in the third trimester. It may also be caused by intra-amniotic haemorrhage, which is subsequently swallowed by the fetus.

It is most commonly seen in the right lower quadrant of the fetus.

Associations

Echogenic bowel can be a normal variant / isolated finding in up to ~70% of cases ^6,12.

Recognised associations include:

• intra-amniotic haemorrhage: with subsequent swallowing of blood products
  • this is considered one of the commonest causes of echogenic fetal bowel, and it may occur after an amniocentesis or placental abruption
• trisomy 21 (~15%) ¹²
  • less commonly associated with trisomy 13, trisomy 18, and Turner syndrome ¹⁴
• intrauterine cytomegalovirus infection: (~15%) ^5,12
• cystic fibrosis
  • detects 2-11% of cystic fibrosis cases ^11,12 
  • may have a higher association if there is concurrent bowel dilatation
• intrauterine growth restriction (IUGR): (~5%) ¹²
• intrauterine fetal demise (IUFD): ~9x increased risk 
  • particularly if serum alpha-fetoprotein levels are also elevated ⁷
  • some data suggests that echogenic bowel occurs before 30 weeks, when there is subsequent IUFD ⁶
• ​meconium peritonitis ¹⁶

Radiographic assessment

Antenatal ultrasound

To be truly 'echogenic bowel' it must be brighter than bone (e.g. femur, spine), and this should be demonstrated on an image with appropriate gain settings. Tissue harmonic imaging and compound imaging should also be switched off.

A sonographic grading system exists to assess the degree of echogenicity ¹⁵, although this is not commonly used in clinical practice.

• grade 0: isoechoic to liver
• grade 1: mildly hyperechoic to liver or less than bone
• grade 2: moderately hyperechoic compared to liver or as echogenic as bone
• grade 3: markedly hyperechoic compared to liver or greater than bone

If there is difficulty discerning whether bowel is as echogenic as bone, one can progressively decrease the image gain to see which structure disappears first.

Treatment and prognosis

Isolated echogenic bowel is associated with a normal outcome in 75% of cases.

Workup of this finding should include, where available, an ultrasound study to assess for any associated anomalies, referral to genetic counselling, TORCH serology, cystic fibrosis carrier testing, and offer of amniocentesis. 

Differential diagnosis

Imaging differential considerations include:

• falsely increased bowel echogenicity due to a high transducer frequency (especially when 8 MHz is used instead of 5 MHz ⁴)
• in the 3^rd trimester, meconium containing bowel may appear echogenic as a normal finding ¹⁰
• meconium peritonitis: coarse calcifications, pseudocysts