|A service of the U.S. National Library of Medicine®|
On this page:
Reviewed November 2014
What is anencephaly?
Anencephaly is a condition that prevents the normal development of the brain and the bones of the skull. This condition results when a structure called the neural tube fails to close during the first few weeks of embryonic development. The neural tube is a layer of cells that ultimately develops into the brain and spinal cord. Because anencephaly is caused by abnormalities of the neural tube, it is classified as a neural tube defect.
Because the neural tube fails to close properly, the developing brain and spinal cord are exposed to the amniotic fluid that surrounds the fetus in the womb. This exposure causes the nervous system tissue to break down (degenerate). As a result, people with anencephaly are missing large parts of the brain called the cerebrum and cerebellum. These brain regions are necessary for thinking, hearing, vision, emotion, and coordinating movement. The bones of the skull are also missing or incompletely formed.
Because these nervous system abnormalities are so severe, almost all babies with anencephaly die before birth or within a few hours or days after birth.
How common is anencephaly?
Anencephaly is one of the most common types of neural tube defect, affecting about 1 in 1,000 pregnancies. However, most of these pregnancies end in miscarriage, so the prevalence of this condition in newborns is much lower. An estimated 1 in 10,000 infants in the United States is born with anencephaly.
What genes are related to anencephaly?
Anencephaly is a complex condition that is likely caused by the interaction of multiple genetic and environmental factors. Some of these factors have been identified, but many remain unknown.
Changes in dozens of genes in individuals with anencephaly and in their mothers may influence the risk of developing this type of neural tube defect. The best-studied of these genes is MTHFR, which provides instructions for making a protein that is involved in processing the vitamin folate (also called vitamin B9). A shortage (deficiency) of this vitamin is an established risk factor for neural tube defects. Changes in other genes related to folate processing and genes involved in the development of the neural tube have also been studied as potential risk factors for anencephaly. However, none of these genes appears to play a major role in causing the condition.
Researchers have also examined environmental factors that could contribute to the risk of anencephaly. As mentioned above, folate deficiency appears to play a significant role. Studies have shown that women who take supplements containing folic acid (the synthetic form of folate) before they get pregnant and very early in their pregnancy are significantly less likely to have a baby with a neural tube defect, including anencephaly. Other possible maternal risk factors for anencephaly include diabetes mellitus, obesity, exposure to high heat (such as a fever or use of a hot tub or sauna) in early pregnancy, and the use of certain anti-seizure medications during pregnancy. However, it is unclear how these factors may influence the risk of anencephaly.
Read more about the MTHFR gene.
How do people inherit anencephaly?
Most cases of anencephaly are sporadic, which means they occur in people with no history of the disorder in their family. A small percentage of cases have been reported to run in families; however, the condition does not have a clear pattern of inheritance. For parents who have had a child with anencephaly, the risk of having another affected child is increased compared with the risk in the general population.
Where can I find information about diagnosis or management of anencephaly?
These resources address the diagnosis or management of anencephaly and may include treatment providers.
General information about the diagnosis and management of genetic conditions is available in the Handbook. Read more about genetic testing, particularly the difference between clinical tests and research tests.
To locate a healthcare provider, see How can I find a genetics professional in my area? in the Handbook.
Where can I find additional information about anencephaly?
You may find the following resources about anencephaly helpful. These materials are written for the general public.
You may also be interested in these resources, which are designed for healthcare professionals and researchers.
What other names do people use for anencephaly?
What if I still have specific questions about anencephaly?
Where can I find general information about genetic conditions?
The Handbook provides basic information about genetics in clear language.
These links provide additional genetics resources that may be useful.
What glossary definitions help with understanding anencephaly?
cerebellum ; cerebrum ; congenital ; deficiency ; diabetes ; diabetes mellitus ; embryonic ; fetus ; fever ; folate ; inheritance ; maternal ; nervous system ; neural tube defects ; pattern of inheritance ; population ; prevalence ; protein ; risk factors ; seizure ; sporadic ; tissue
You may find definitions for these and many other terms in the Genetics Home Reference Glossary.
See also Understanding Medical Terminology.
References (9 links)
The resources on this site should not be used as a substitute for professional medical care or advice. Users seeking information about a personal genetic disease, syndrome, or condition should consult with a qualified healthcare professional. See How can I find a genetics professional in my area? in the Handbook.