Anencephaly is the absence of a large part of the brain and the skull.

Alternative Names

Aprosencephaly with open cranium


Anencephaly is a neural tube defect that occurs early in the development of an unborn baby. Neural tube defects involve the tissue that grows into the brain and spinal cord. Anencephaly results when the upper portion of the neural tube fails to close. Why this happens is not known. Possible causes include environmental toxins and low intake of folic acid during pregnancy.

Anencephaly occurs in about 4 out of 10,000 births. The exact number is unknown, because many of these pregnancies spontaneously abort. Having one anencephalic infant increases the risk of having another child with neural tube defects.


In the mother:

  • If there is too much fluid is in the uterus, this often suggests a problem with the pregnancy. This condition, called polyhydramnios, is seen before birth.
In the infant:
  • Absence of the skull
  • Absence of the brain (cerebral hemispheres and cerebellum)
  • Facial feature abnormalities
  • Heart defects

Exams and Tests

The following tests can help identify anencephaly:

  • Prepregnancy serum folic acid test
  • Amniocentesis (done on the mother to determine if increased levels of alpha-fetoprotein are present)
  • Alpha-fetoprotein levels during pregnancy (increased levels suggest a neural tube defect)
  • Urine estriol levels during pregnancy
  • Ultrasound to confirm the diagnosis


No specific therapy is recommended, since this is a fatal condition.

Outlook (Prognosis)

This condition is usually fatal within days.

When to Contact a Medical Professional

This condition is usually found by the physician during routine prenatal testing and ultrasound. Otherwise, it is recognized at birth.

If anencephaly is detected before birth, further counseling will be needed.


It is important for women who may become pregnant to get enough folic acid. To reduce the risk of some neural tube defects, talk to your doctor about getting the right amount of folic acid for at least three months before becoming pregnant and during the first month of pregnancy.

Getting enough folic acid can reduce the chance of neural tube defects by 50 percent.

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