Exencephaly is early manifestation of anencephaly before neural tissue has worn away

No calvarium with absence of neural tissue above orbits

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  Neural tissue wears away during gestation until no organized neural tissue remains

  
  
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  Cranial defect covered by angiomatous stroma, which gives “lumpy” configuration in 2nd trimester

  
  
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  Often contiguous with cervical spine defect

Should be able to diagnose routinely at 10-14 weeks

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  Neural tissue often still present (exencephaly)

  
  
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  Head has irregular, flattened, splayed appearance

Proptotic eyes (described as frog-like appearance)

Amniotic fluid often echogenic secondary to dissolved neural tissue

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  Polyhydramnios common later in pregnancy

Amniotic band syndrome

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  Calvarium may be absent, but large amount of intact brain often remains

  
  
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  Look for other defects and presence of fine linear bands

  
  
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  Important distinction for counseling, as bands are sporadic and have no increased recurrence risk

  
  
  
  
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    Exencephaly, anencephaly has 2-5% recurrence risk, and preconceptual folic acid treatment needed for next pregnancy

  
  

CRL < expected is not always due to incorrect dates

Endovaginal scanning in 1st trimester for earlier diagnosis

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  May still be difficult diagnosis to make before 10 weeks

  
  
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  Visible tissue of exencephaly may be mistaken for normal brain, so examine cranial contour carefully

Short-term follow-up for any case when head looks asymmetric or irregular