Inability to demonstrate normal anal dimple or target sign

Normal appearance

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  Axial: Hyperechoic mucosa within hypoechoic muscular ring

  
  
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  Coronal: Hyperechoic mucosal stripe between hypoechoic walls, extending to perineum

  
  
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  No other appearance is proof of normal anorectal development; should be demonstrable from 20 weeks

Dilated, fluid-filled distal bowel is late manifestation; not seen at time of 2nd-trimester survey

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  U- or V-shaped bowel in presacral space without extension to perineum

Polyhydramnios unusual

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  Unlike proximal small bowel atresias

Remember that colon is often prominent in 3rd trimester

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  Normal caliber ≤ 18 mm

3D data sets can be obtained to demonstrate rectum in sagittal and coronal planes

Look for calcified meconium enteroliths

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  Results from mixture of meconium and fetal urine within bowel lumen if fistula between colon and bladder

  
  
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  Look for echogenic “marbles” moving within bowel

Look for other abnormalities as anal atresia is seldom isolated

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  VACTERL association ( v ertebral, a nal atresia, cardiac, t racheo- es ophageal, r enal and l imb anomalies)

  
  
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  Cloacal exstrophy

  
  
  
  
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    Abdominal wall defect, absent bladder, often spine abnormalities

  
  
  
  
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  Cloacal malformation

  
  
  
  
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    Female fetus, distended vagina, small bladder

  
  

Take dedicated views of rectum/anus when any bowel, spine, or genital abnormality is present