Birth Definitions

Anencephaly Definition


Anencephaly is a neural tube defect (NTD) that occurs when the rostral (head) end of the neural tube fails to close properly during embryonic development. The neural tube is a layer of cells that forms during early development and helps to create the brain and spinal cord. When this tube will not close properly, it results in defects such as the absence of a major portion of the brain, skull and scalp. Anencephaly is a type of cephalic disorder.

Babies with anencephaly are born without a brain or with a brain that is not fully developed. There is no cure for anencephaly and most babies with this condition die within a few hours or days after birth.

Types of Anencephaly

The most common type of anencephaly is alobar, in which there is complete absence of the forebrain and cerebrum. The second type, lobar anencephaly, is characterized by partial agenesis of the forebrain. The third and rarest type, merosynaptic anencephaly, involves failure of closure of only a portion of the neural plate.


The cause of anencephaly is unknown. It is thought to be caused by a combination of genetic and environmental factors. Some women who have a family history of neural tube defects are at increased risk for having a baby with anencephaly. Women who take certain medications during pregnancy, such as certain anticonvulsants, are also at increased risk. There is some evidence that folic acid deficiency may play a role in its development. Women who take folic acid supplements during pregnancy can reduce their risk of having a child with anencephaly by up to 70%.

Diagnosis and Related Terms

Anencephaly can be diagnosed during pregnancy with a prenatal ultrasound. If anencephaly is diagnosed, the parents may choose to end the pregnancy or continue the pregnancy and provide comfort care for the baby after birth.

There are a number of terms that are closely associated with anencephaly, including neural tube defect, birth defect, and congenital anomaly.

This image shows how often the term ‘Anencephaly’ is used in relation to other, similar birth terms:

Other Related Terms

The following are other terms closely related to the word Anencephaly:

  • Agenesis of the corpus callosum
  • Arnold-Chiari malformation
  • Cephalic disorder
  • Neural tube defect (NTD)

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