Anencephaly is one of the most common neural tube defects. Neural tube defects are birth defects that affect the tissue that becomes 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. The exact cause is not known. Possible causes include:

• Environmental toxins
• Low intake of folic acid by the mother during pregnancy

The exact number of cases of anencephaly is unknown. Many of these pregnancies result in miscarriage. Having one infant with this condition increases the risk of having another child with neural tube defects.

Symptoms of anencephaly are:

• Absence of the skull
• Absence of parts of the brain
• Facial feature abnormalities
• Severe developmental delay

Heart defects may be present in 1 out of 5 cases.

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

The mother may also have these tests during pregnancy:

• 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.

There is no current treatment. Talk to your health care provider about care decisions.

This condition most often causes death within a few days after birth.

A provider usually detects this condition during routine prenatal testing and ultrasound. Otherwise, it is recognized at birth.

If anencephaly is detected before birth, further counseling will be needed.

There is good evidence that folic acid can help reduce the risk for certain birth defects, including anencephaly. Women who are pregnant or planning to become pregnant should take a multivitamin with folic acid every day. Many foods are now fortified with folic acid to help prevent these kinds of birth defects.

Getting enough folic acid can cut the chances of neural tube defects in half.

Aprosencephaly with open cranium

Dean B, Doherty D. Congenital malformations of the central nervous system. In: Gleason CA, Sawyer T, eds. Avery's Diseases of the Newborn. 11th ed. Philadelphia, PA: Elsevier; 2024:chap 53.

Kinsman SL, Johnston MV. Congenital anomalies of the central nervous system. In: Kliegman RM, St. Geme JW, Blum NJ, Shah SS, Tasker RC, Wilson KM, eds. Nelson Textbook of Pediatrics. 21st ed. Philadelphia, PA: Elsevier; 2020:chap 609.

Sarnat HB, Flores-Sarnat L. Developmental disorders of the nervous system. In: Jankovic J, Mazziotta JC, Pomeroy SL, Newman NJ, eds. Bradley and Daroff''s Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022:chap 89.

Updated by: Neil K. Kaneshiro, MD, MHA, Clinical Professor of Pediatrics, University of Washington School of Medicine, Seattle, WA. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.