Gastrointestinal (GI) tract forms from 1 straight tube

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  Foregut : Forms esophagus, stomach, and duodenum

  
  
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  Midgut : Small intestine and colon up to splenic flexure

  
  
  
  
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    Connected to yolk sac

    
    
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    Portion that lengthens and loops around superior mesenteric artery (SMA)

  
  
  
  
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  Hindgut : Descending colon, sigmoid, rectum, and upper anal canal

  
  
  
  
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    Caudal end of hindgut terminates in cloaca

    
    
    
    
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      Cloaca (Latin for sewer) is common chamber with early communication between the urinary, GI, and reproductive tracts

    
    

  
  

Physiologic herniation

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  Length of midgut increases rapidly and is greater than body can accommodate so it herniates into base of umbilical cord

  
  
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  It rotates 90°counterclockwise around axis of SMA and returns to the abdomen after total rotation of 270°

  
  
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  Abdominal wall closes around base of cord (umbilical ring)

  
  
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  Physiologic hernia is commonly seen on early 1st trimester scans but the bowel should be back within the abdomen by 12 weeks gestational age

  
  
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  An omphalocele results from failure to complete physiologic herniation or close umbilical ring

Rectum forms when urorectal septum divides cloaca into rectum posteriorly and urogenital sinus anteriorly

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  Cloacal membrane ruptures by beginning of 8th week, creating anal opening

  
  
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  Urogenital sinus will divide to form bladder and in females, the vagina

American Institute of Ultrasound in Medicine (AIUM) scan of the abdomen requires documentation of stomach, kidneys, bladder, umbilical cord insertion site, and umbilical cord vessel number

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  Diaphragm, esophagus, small intestine, colon, gallbladder, and liver should also be examined but is not required as part of the standard midtrimester scan

Stomach is seen as a fluid-filled structure in the left upper quadrant and is one of the 1st organs identified

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  Document heart and stomach are on same side, and it is anatomic left of fetus (normal situs)

  
  
  
  
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    Opposite sides are seen in heterotaxy syndromes

  
  
  
  
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  Changes in size and shape during exam

  
  
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  Fluid may intermittently be seen to enter the duodenal bulb but should never persist

  
  
  
  
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    A persistently dilated duodenum is never normal and suspicious for duodenal atresia

  
  

Fluid must be visualized on both sides of the umbilical cordinsertion in a transverse section of the fetal abdomen

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  Stimulation of fetal movement may be necessary to create a more favorable acoustic window, especially in 3rd trimester when the fetal knees are often tucked up against abdominal wall

Normal cord contains 2 arteries and 1 vein

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  May be visible at cord insertion site, but easiest way to confirm is color Doppler showing umbilical arteries on either side of bladder

Diaphragm appears as a thin, arched, hypoechoic band

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  It is imperative that it be completely imaged from front to back, which is best done in the sagittal plane

  
  
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  If viewed only in the anterior coronal plane, a congenital diaphragmatic hernia (CDH) may be missed

Esophagus is not normally seen on fetal imaging, but a blind-ending, fluid-filled pouch may be seen in the fetal neck in esophageal atresia

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  Use color Doppler to ensure that the fluid-filled structure is between the neck vessels

Bowel

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  In early 2nd trimester, often appears as intermediate echogenicity “filler” between the solid organs, bladder, and stomach; higher frequency transducers may show distinct bowel loops

  
  
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  Normal meconium-filled colon often prominent near term

  
  
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  Anal dimple best seen on an axial view of perineum

  
  
  
  
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    Anal mucosa is echogenic and surrounded by hypoechoic muscles of the anal sphincter, creating a target or doughnut appearance

  
  

Fetal liver is relatively large and extends across the upper abdomen with the left lobe anterior to stomach

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  Major contributor to the abdominal circumference (AC)

  
  
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  Both portal and hepatic veins seen on color Doppler

  
  
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  Gallbladder may be seen, especially in the 3rd trimester, and should not be confused with an abdominal cyst

Is the abdominal wall intact?

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  Gastroschisis (most common abdominal wall defect) is generally located to the right of the umbilical cord insertion and is not covered by a membrane

  
  
  
  
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    Small bowel is the most commonly extruded organ, although stomach, large bowel, and other structures may also be involved

  
  
  
  
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  Omphalocele involves extrusion of bowel into the base of the umbilical cord

  
  
  
  
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    Covered by a membrane; umbilical cord inserts on this membrane

    
    
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    May rarely rupture; in these cases, it may be difficult to distinguish from gastroschisis

  
  
  
  
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  Defects may also occur in more unusual locations

  
  
  
  
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    Low, suprapubic mass may be associated with bladder or cloacal exstrophy

    
    
    
    
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      Both will have absent bladder, but cloacal exstrophy will also have extruded bowel described as appearing like an elephant trunk

    
    
    
    
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    Supraumbilical defect associated with diaphragmatic and cardiac abnormality is seen in pentalogy of Cantrell

    
    
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    Other unusual or bizarre abdominal wall defects may be seen in cases of amniotic bands or body stalk anomaly

  
  

Is the fetus freely mobile?

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  In body stalk anomaly , the fetus is “stuck” to the placenta, and the umbilical cord is absent or very short

  
  
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  A fetus entrapped within amniotic bands may also be tethered in one position

  
  
  
  
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    Look for strands of membrane or other defects, such as unusual facial or cranial clefts

  
  

Is the abdomen normal in size?

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  Per AIUM guidelines, the AC is measured at the skin line on a true transverse view at the level of the junction of the umbilical vein, portal sinus, and fetal stomach

  
  
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  AC is utilized with other biometric parameters to calculate the fetal weight/average gestational age

  
  
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  AC below the normal range

  
  
  
  
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    Generally, the most affected parameter in growth restriction

    
    
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    May also measure small when normal abdominal contents are outside the abdomen (e.g., gastroschisis, omphalocele) or up in the chest (i.e., CDH)

  
  
  
  
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  AC above the normal range

  
  
  
  
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    Macrosomic fetus of a diabetic mother

    
    
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    Overgrowth syndromes, such as Beckwith-Wiedemann, may also exhibit increased AC size, primarily due to enlarged kidneys and liver

    
    
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    AC often increased in fetuses with large abdominal masses, dilated bowel, or distended bladder

  
  

Is the stomach normal?

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  A fluid-filled stomach should reliably be identified after 14 weeks

  
  
  
  
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    If not seen, short-term follow-up required to confirm its presence or absence

    
    
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    Ensure that it is not in an abnormal location, such as within the chest in a CDH

  
  
  
  
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  Small/absent stomach

  
  
  
  
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    Esophageal atresia ± tracheoesophageal fistula

    
    
    
    
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      Look for blind-ending pouch in neck

      
      
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      Will have significant polyhydramnios by 3rd trimester

    
    
    
    
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    May be seen in cases of decreased swallowing (e.g., neurologic disorder)

  
  
  
  
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  Large stomach

  
  
  
  
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    Often a transient finding or may be seen in evolving, distal GI obstructions

    
    
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    Double bubble sign (dilated stomach and duodenum) is seen in duodenal atresia

  
  

Is there an abdominal mass?

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  Masses should be characterized as to their location and appearance (cystic, solid, or complex; vascular or nonvascular) to narrow the differential diagnosis

  
  
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  Cystic masses in the abdomen are relatively common

  
  
  
  
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    Many of these are related to the urinary tract and include cystic kidneys, lower urinary tract obstruction, and ovarian cysts

    
    
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    GI causes include

    
    
    
    
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      Bowel atresia : Look for peristalsis

      
      
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      Meconium pseudocyst : Irregular cystic mass, which forms after bowel perforation; look for other sequelae, including peritoneal calcifications

      
      
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      Enteric duplication cyst : Look for gut signature

      
      
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      Mesenteric cysts/lymphangioma

      
      
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      Persistent cloaca occurs when genitourinary tract and colon never separate

    
    

  
  
  
  
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  Solid masses are less common; the differential diagnosis starts with the organ of origin

  
  
  
  
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    The most common liver mass is a congenital hemangioma , which usually has prominent vascularity

    
    
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    Bulk of a sacrococcygeal teratoma is exophytic but may extend into pelvis/abdomen

    
    
    
    
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      Rarely may be only intrapelvic with no external component

    
    
    
    
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    Fetus-in-fetu is a mixed solid/cystic mass and is often quite large

    
    
    
    
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      Calcifications common; bones and vertebrae may be seen

    
    

  
  

Are there calcifications in the abdomen?

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  Calcifications on the surface of the liver are actually in the peritoneum

  
  
  
  
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    These correlate strongly with intrauterine bowel perforation

    
    
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    Look for associated echogenic or dilated bowel loops, small amounts of ascites, &/or meconium pseudocysts to add weight to this diagnosis

  
  
  
  
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  Calcification in the liver parenchyma concerning for infection, most commonly cytomegalovirus

  
  
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  Calcifications in the bowel lumen indicate admixture of meconium and urine in the setting of abnormal distal bowel and bladder development

  
  
  
  
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    These “meconium marbles” roll within the bowel lumen with peristalsis

    
    
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    Look carefully for the anal dimple to detect associated anal atresia

  
  

Does the bowel appear echogenic?

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  A high-frequency transducer may give the false impression of echogenic bowel

  
  
  
  
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    Confirm the finding is persistent with a lower frequency transducer (< 5 MHz)

  
  
  
  
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  Bowel is not abnormal unless it is as bright as bone

  
  
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  Fetal ingestion of blood from a recent bleed is a common benign cause and resolves spontaneously

  
  
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  Evaluate for pathologic causes, including aneuploidy, infection, cystic fibrosis , and early bowel abnormalities, such as atresia , before the bowel becomes dilated

  
  
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  May be seen in bowel ischemia in association with severe growth and hemodynamic stress as in twin-twin transfusion

Is there ascites?

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  Care should be taken to differentiate true ascites from pseudoascites , a potential pitfall created by the hypoechoic abdominal wall musculature

  
  
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  Ascites may be 1st sign of impending hydrops

  
  
  
  
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    Look for other evidence of hydrops (pleural and pericardial effusions and skin edema)

  
  
  
  
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  Chest masses may compromise venous and lymphatic return and cause isolated ascites without generalized hydrops

  
  
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  May also result from perforation of an abdominal viscus , either bowel or bladder