Anencephaly results from lack of closure of the cephalic end of the neural tube (rostral or cephalic neuropore) at approximately 3-4 weeks of embryological development. Normal migration of mesenchymal tissue in this region does not therefore occur, leading to effective absence of the cranial vault and progressive loss of functioning cerebral tissue, although portions of the midbrain and brain stem, along with remnants of the bony cranium, may be present. A membrane of angiomatous stroma (area cerebrovasculosa) covers the neural tube defect allowing leakage of alpha fetoprotein across the defect into the amniotic fluid and into the maternal serum. Anencephaly is thought to represent the end stage of a process seen earlier in exencephaly, resulting from defective rostral neurulation.


Anencephaly can be diagnosed with virtually 100% accuracy on sonograms performed after 14 menstrual weeks. It is characterised by symmetric absence of normally formed brain and skull superior to the orbits. The presence of varying amounts of angiomatous stroma may be noted but does not improve the uniformly poor prognosis of these fetuses. Associated anomalies of the central nervous system are common. 50% of anencephalic fetuses have concurrent spinal defects, notably meningocele and myelomeningocele. Other structural abnormalities include hypoplastic or absent adrenal glands, hydronephrosis, talipes equinovarus, omphalocele, cleft lip and palate. Polyhydramnios may be noted in 30-50% of cases, typically after 26 weeks. 

Differential Diagnosis

Amniotic band syndrome presents with asymmetric fetal head destruction and should not be confused with the symmetric lesion of anencephaly. There may be other signs present, such as limb and digit amputations, which will enable the correct diagnosis. In cases of severe microcephaly a cranial vault can always be distinguished while in anencephaly it is always absent.

Sonographic Features

Symmetric absence of cranial vault and cerebral cortex. Varying amount of residual brain may be present (area cerebrovasculosa), especially in early pregnancy Orbits and base of skull present Polyhydramnios in 50% Severe rachischisis common.

Associated Syndromes

  • Aminopterin
  • Amniotic band
  • Brachydactyly
  • Chromosomal
  • Facial duplication
  • Maternal hyperthermia
  • Prune belly
  • Roberts-SC phocomelia
  • Short rib polydactyly
  • Thoracoabdominal eventration
  • X-linked midline defects
  • X-linked neural tube defects


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