What is anencephaly?

Anencephaly, also known as "open skull," is a birth defect in which the major parts of the brain, scalp, and skull of the fetus do not form completely as it is developing in the womb.

Anencephaly occurs when the neural tube, a narrow channel in the fetus that normally closes to form the spinal cord and brain, does not close properly between the third and fourth weeks of pregnancy. An infant born with anencephaly has some or most of the brain missing. These infants are unconscious, cannot feel, and are usually blind and deaf.

In many cases, some brain tissue may be exposed because parts of the skull are missing. Some infants may have a primitive brain stem at birth. They may seem to respond to sound or touch, but their reactions are involuntary and are caused by the action of the brain stem.

Anencephaly is a fatal condition. Infants with anencephaly are stillborn in about 75 percent of cases. Newborns who survive die within several hours, days, or weeks.

How common is anencephaly?

About one in nearly 5,000 babies is born with anencephaly each year, according to the Centers for Disease Control. The exact number is not known because many pregnancies involving neural tube defects end in miscarriage. More female newborns have anencephaly than males, possibly because of a higher rate of spontaneous abortions or stillbirths among male fetuses.

What causes anencephaly?

The exact causes of anencephaly are unknown. A lack of folic acid (vitamin B9) before and during pregnancy may increase the chance of neural tube defects, such as anencephaly or spina bifida (a birth defect in which the spine is exposed). If a woman takes certain prescription drugs during pregnancy, such as drugs for diabetes, this may increase the risk of having an infant with anencephaly.

In about 90 percent of cases, the parents of an anencephalic infant do not have a family history of the disorder. However, if the parents have had a child who was born with anencephaly, they have a greater chance of having another baby with this condition. The recurrence rate for anencephaly is 4 to 5 percent, and rises to 10 to 13 percent if the parents have had two other children with anencephaly.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy