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What is anencephaly?

Anencephaly is a severe congenital (present at birth) condition that occurs when a fetus’s brain, skull and scalp don’t develop properly. Parts of the brain and skull are missing, leaving the brain tissue that forms exposed and unprotected.

Congenital conditions like anencephaly that affect the brain, spinal cord and nerves are called neural tube defects (NTDs). The neural tube is tissue in a fetus that becomes the brain, spinal cord and spine. An NTD occurs if the tube doesn’t close properly when it forms during the first month of pregnancy. Anencephaly occurs when the top part of the neural tube doesn’t close completely.

Other abnormalities of the fetal skull include:

  • Meroacrania: In this milder form, a smaller area of the skull is missing.
  • Acrania: The entire skull is missing, and the brain is not fully formed.
  • Holoanencephaly: This is the most severe type, in which the brain is completely missing.

Babies who have anencephaly have a higher chance of being born prematurely or being stillborn. Unfortunately, most full-term babies who have anencephaly pass away within a few hours or days after birth.

What causes anencephaly?

The causes of anencephaly aren’t fully understood, but researchers believe that a combination of genetic and environmental factors may be involved. Most cases of anencephaly occur in families with no history of the condition, so it doesn’t appear to be an inherited condition.

Certain factors may increase a mother’s risk of having a baby with anencephaly. Not everyone with risk factors will have a baby with anencephaly, and some women without risk factors can have a baby with the condition.

The risk factors for anencephaly include:

  • Folic acid (vitamin B9) deficiency before and during pregnancy
  • High heat exposure during pregnancy, such as having a fever or using a hot tub or sauna
  • Certain medications that treat seizures, migraine headaches and bipolar disorder
  • Opioid use during pregnancy
  • Uncontrolled diabetes before pregnancy

Taking a 400-microgram (4 milligrams) supplement of folic acid every day, before and during pregnancy, can greatly reduce the risk of having a baby with anencephaly.

Who gets anencephaly?

Anencephaly makes up about half of all cases of neural tube defects. According to the Centers for Disease Control and Prevention, about one in 4,600 babies is born with anencephaly each year in the United States.

Female babies are three times more likely to develop anencephaly than male babies. Hispanic women are more likely than other ethnicities to have a baby with anencephaly.

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