What Is It, Risk Factors, Prevention and More

Author:Lahav Constantini

Editors:Alyssa Haag,Józia McGowan, DO,Kelsey LaFayette, DNP, ARNP, FNP-C

Illustrator:Jessica Reynolds, MS

Copyeditor:Stacy Johnson, LMSW

What is anencephaly?

Anencephaly is a rare but serious and fatal condition of development in which a baby is born without the majority of its brain, skull, and scalp. The name comes from the prefix ‘an’, which means ‘without,’ and the Greek word ‘enkephalos,’ which means ‘brain.’ This birth defect occurs during the early pregnancy phases and can be diagnosed during pregnancy with screening tests or immediately after birth with a physical examination. Unfortunately, a newborn with anencephaly usually only survives for several hours or days. 

Anencephaly is a neural tube defect (NTD). The neural tube is an embryonic structure from which the fetus's nervous system develops. It typically closes on the 4th week of pregnancy. When the rostral part (i.e., toward the head) of the neural tube doesn’t close appropriately, anencephaly occurs, which means that parts of the brain and the skull bones don’t develop as usual. On the other hand, when the neural tube's caudal region (i.e., toward the tail) doesn’t properly close, spina bifida occurs. In spina bifida, the signs and complications range from mild when there are tiny gaps in the vertebral column (i.e., spina bifida occulta) to more severe when there is a protrusion of the spinal cord through the spinal column (i.e., myelomeningocele).

Infant missing the majority of its brain, skull, and scalp.

What causes anencephaly?

Anencephaly is believed to result from various nutritional, genetic, and environmental factors rather than a single cause. Nonetheless, the exact underlying mechanism is unknown. The most critical risk factor during pregnancy is a lack of folic acid or vitamin B9. Since certain foods are now fortified with folic acid, the incidence of neural tube defects has dramatically decreased. Other specific risk factors for anencephaly include female sex of the infant; maternal obesity; Hispanic ethnicity; pregestational diabetes mellitus or gestational diabetes; multiple or prolonged saunas or hot tub use; and taking certain teratogenic drugs, such as valproate (i.e., a medication used to treat epilepsy and bipolar disorder). It has been hypothesized that genetics play a part in the pathophysiology of anencephaly as there are variations of prevalence between different ethnic groups, as well as a higher incidence in parents who had previous pregnancies with neural tube defects. The season at which conception occurs and other geographical variations may also play a role in the pathogenesis of anencephaly.

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What are the signs and symptoms of anencephaly?

Newborns with anencephaly are born without parts of their brain tissue, skull, and scalp. The superior part of their head is typically absent or particularly small, and some of their brain tissue may be exposed. They’re commonly born unconscious, blind, deaf, and without pain sensitivity. In some cases, if a rudimentary part of the brainstem evolves, the infant can have some reflex actions, such as breathing or responding to touch or sound. 

In addition, the parts of the fetal brain that are responsible for neural control of swallowing are absent, so the fetus is unable to swallow amniotic fluid while in utero. A consequent fluid accumulation in the amniotic sac, called polyhydramnios, increases the risk of pregnancy complications, such as fetal malpositioning, placental abruption, and premature birth

How is anencephaly diagnosed?

Anencephaly is linked to higher-than-expected levels of alpha-fetoprotein (AFP) in the blood. Anencephaly can be identified when the fetus is still in the uterus using prenatal screening tests, such as maternal serum markers (e.g., alpha-fetoprotein) and ultrasonography, which are usually done in the second trimester as part of routine monitoring. On ultrasound, anencephaly may present with the associated structural abnormalities, such as the absence of brain tissue and skull. To diagnose anencephaly after delivery, a physical examination is done to ascertain the structural deformities based on these criteria; absence of calvarium (i.e., the top part of the cranium), absence of the scalp, absence of the cerebral hemispheres, and the presence of tissue external to the skill or a mass that is hemorrhagic and fibrous.

How is anencephaly treated?

Currently, anencephaly has no cure or treatment. Without a functioning cerebrum, infants are stillborn or die within a few hours or days after birth, which makes prevention necessary to reduce the incidence of anencephaly and neural tube defects

While NTDs cannot be entirely prevented, several risk factors can be addressed to decrease the risk and ensure early detection and diagnosis. One important preventative measure includes taking folic acid supplements in the first 12 weeks of pregnancy. The Centers for Disease Control (CDC) also recommends that those assigned female at the birth of reproductive age receive 400 mcg of folic acid every day, regardless of current pregnancy or planning a pregnancy, as many individuals have unplanned pregnancies or may not know that they are pregnant until it is later in their pregnancy. 

Other risk factors that can be addressed to reduce the incidence of anencephaly include limiting the use of hot tubs, managing obesity and diabetes with lifestyle and medical management, and routine check-ups. It’s especially recommended for those taking anticonvulsant medications before starting a family to undergo counseling to understand their risk of seizures during pregnancy, optimize their medication dosage, and construct a proper plan with their healthcare provider. 

What are the essential facts to know about anencephaly?

Anencephaly is a fatal birth defect and one of the main neural tube defects, in which an infant is born without most of the brain and its overlying skull and scalp. This typically results in the death of the infant shortly after birth. Anencephaly is an embryonic development defect that occurs very early in pregnancy when the neural tube doesn’t properly close. This typically results in the death of the infant shortly after birth. While the exact underlying cause is yet to be known, several genetic, environmental, geographical, and nutritional risk factors are associated, which include Hispanic ethnicity; female sex of the infant; obesity and diabetes history in the pregnant individual; use of hot tubs; lack of folic acid supplementation; and certain medications, such as anti-seizure drugs. These risk factors can be addressed and managed to reduce the risk of anencephaly. Diagnosis can be done during pregnancy, mainly through ultrasound and increased alpha-fetoprotein in maternal serum or right after physical examination. 

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Related links

Spina bifida
Folate (Vitamin B9) deficiency
Congenital neurological disorders: Pathology review

Resources for research and reference

Agopian, A. J., Tinker, S. C., Lupo, P. J., Canfield, M. A., Mitchell, L. E., & National Birth Defects Prevention Study (2013). Proportion of neural tube defects attributable to known risk factors. Birth defects research. Part A, Clinical and molecular teratology, 97(1), 42–46.

Duong, H. T., Shahrukh Hashmi, S., Ramadhani, T., Canfield, M. A., Scheuerle, A., Kim Waller, D., & National Birth Defects Prevention Study (2011). Maternal use of hot tub and major structural birth defects. Birth defects research. Part A, Clinical and molecular teratology, 91(9), 836–841.

Marcdante, K. J., & Kliegman, R. (2019). Nelson essentials of pediatrics (Eighth edition.). Philadelphia, PA: Elsevier.

Padmanabhan R. Etiology, pathogenesis, and prevention of neural tube defects. Congenit Anom (Kyoto). 2006 Jun;46(2):55-67.

Tafuri SM, Lui F. Embryology, Anencephaly. [Updated 2022 May 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from:

Yamaguchi Y, Miyazawa H, Miura M. Neural tube closure and embryonic metabolism. Congenit Anom (Kyoto). 2017 Sep;57(5):134-137.