Anencephaly is the absence of a large part of the brain and the skull.

Anencephaly is one of the most common neural tube defects. Neural tube defects are birth defects that affect the tissue that grows into the spinal cord and brain. Anencephaly occurs early in the development of an unborn baby. It results when the upper part of the neural tube fails to close. Why this happens is not known. Possible causes include environmental toxins and low intake of folic acid during pregnancy.

Anencephaly occurs in about 4 out of 10,000 births. The exact number is unknown, because many of these pregnancies result in miscarriage. Having one infant with this condition increases the risk of having another child with neural tube defects.

• Absence of the skull
• Absence of the brain (cerebral hemispheres and cerebellum)
• Facial feature abnormalities
• Heart defects

A pregnancy ultrasound is done to confirm the diagnosis. The ultrasound may reveal too much fluid is in the uterus. This condition is called polyhydramnios.

Other tests that may be done on the pregnant mother:

• Amniocentesis (to look for increased levels of alpha-fetoprotein)
• Alpha-fetoprotein level (increased levels suggest a neural tube defect)
• Urine estriol level

A pre-pregnancy serum folic acid test may also be done.