What is anencephaly?

Anencephaly is a birth defect (a health problem identified at birth). It occurs when the skull, scalp and brain do not develop properly in the womb. Portions of the baby’s brain and skull are missing. The brain tissue that does form is usually exposed because there isn’t enough skin and bone to cover it.

Birth defects in the nervous system (the brain, spine and nerves), like anencephaly, are neural tube defects (NTDs). Neural tube problems develop very early in pregnancy. Babies born with anencephaly live only a few hours or days after birth.

How common is anencephaly?

About one out of every 5,000 to 10,000 babies is born with anencephaly, and the condition affects baby girls more often than boys. Most pregnancies with anencephaly end in miscarriage or stillbirth. Women who have had another child with an NTD, such as spina bifida, have a higher risk of conceiving a child with anencephaly.

What causes anencephaly?

Anencephaly doesn’t appear to be inherited (passed down in families). In most cases, it occurs without any family history of the condition. But if you’ve had a child with a neural tube defect (NTD) before, you have a higher chance of having a baby with anencephaly.

A combination of environmental factors, genes and nutrition during pregnancy likely causes anencephaly. Certain drugs and risk factors increase the chance of having a baby with anencephaly or another NTD, including:

  • Lack of folic acid: Women who don’t get enough folic acid (vitamin B9) when they’re pregnant have a higher risk of having a baby with anencephaly. Women should take a prenatal vitamin with 400 micrograms (mcg) of folic acid before and during pregnancy.
  • Diabetes: Uncontrolled diabetes increases the risk of NTDs. It causes blood glucose levels (sugar in the blood) to get too high and harm your baby’s development.
  • High body temperature: Having a fever or using a hot tub or sauna during early pregnancy can increase your risk of having a baby with an NTD.
  • Medications: Anti-seizure drugs such as phenytoin (Dilantin®), carbamazepine (Tegretol®) and valproic acid (Depakote®) can cause NTDs. Some of these drugs also treat migraines and bipolar disorder.
  • Obesity: Women who carry excess weight before pregnancy have a higher chance of having a baby with anencephaly or another NTD.
  • Opioid use: Taking opioids during the first two months of pregnancy can cause NTDs. Opioids include heroin (an illegal drug) and prescription painkillers such as hydrocodone.

How does anencephaly occur?

Sometimes called “open skull,” anencephaly happens when the upper part of the neural tube doesn’t close completely during the baby’s development. The neural tube is a flat piece of tissue that grows into a tube and forms the brain and spinal cord. Without a closed tube, the brain and skull don’t develop.

Like all neural tube defects, anencephaly occurs during the third and fourth weeks of pregnancy. The rest of the baby’s body continues to form and grow as the pregnancy progresses.

Last reviewed by a Cleveland Clinic medical professional on 08/18/2020.

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