NORD gratefully acknowledges Cynthia A. Moore, MD, PhD, and Phoebe G. Thorpe, MD, MPH, Division of Birth Defects and Developmental Disabilities, National Center on Birth Defects and Developmental Disabilities, Centers for Disease Control and Prevention, for assistance in the preparation of this report.
Anencephaly is a term that refers to the incomplete development of the brain, skull, and scalp and is part of a group of birth defects called neural tube defects (NTD). The structure which will become the neural tube is supposed to fold and to close together (to form a tube) during the third and fourth weeks of pregnancy. From this neural tube, the brain and spinal cord of the embryo develop. Neural tube defects happen when the neural tube does not close as expected. Anencephaly occurs when the end of the neural tube that would have developed into the brain does not close properly, resulting in the failure of the development of major portions of brain, skull and scalp. Other neural tube defects, such as spina bifida, form when the neural tube does not close properly in a different part of the neural tube.
Infants with anencephaly are born without the front part of the brain, (forebrain) and the thinking and coordinating part of the brain (cerebral hemispheres and cerebellum). Most of the time the remaining brain tissue may be exposed, without skull or scalp to cover and protect it. Although reflex actions such as breathing and responses to touch or sound may occur, gaining consciousness is not possible. Usually infants with anencephaly do not survive more than a few days or weeks.
Meroanencephaly and holoanencephaly are terms refer to the extent of the cranial defect, however, they typically are not used in clinical descriptions and are not predictive of severity of the condition. The term acrania has been used interchangeably with anencephaly in some parts of the world but that practice is discouraged as it confuses two very different conditions.
Pregnancies affected by anencephaly show high levels of a specific fetal protein (called alpha fetoprotein) in tests done on the mother’s blood or on amniotic fluid. The presence of excess fluid in the amniotic sac that contains the fetus (polyhydramnios) may be noted during an ultrasound.
Anencephaly may also be seen before birth using any one of several imaging techniques. The condition is characterized by the absence of the skull and parts of the brain (cerebral hemispheres and cerebellum). Abnormalities of facial features secondary to the absent skull are common and vary depending on the development of each fetus. Anencephaly usually (~80%) occurs without other birth defects.
Not all of the causes of anencephaly are understood. Anencephaly can be a multifactorial condition meaning that multiple genes are involved interacting with environmental agents and chance events to cause the condition. Anencephaly can also be a feature of some chromosomal disorders such as trisomy 18 which are usually sporadic and not familial (inherited).
Most times anencephaly occurs in a pregnancy where there is no family history of neutral tube defects. However, research has shown that once a woman has had one pregnancy that resulted in a fetus with a neural tube defect, any additional pregnancies have an increased risk that a similar defect will occur again (recurrence risk). This recurrence risk is estimated to be 3-4% compared to the background risk of much less than 1%. Researchers also expect that if a subsequent fetus has a neural tube defect, the defect, whether anencephaly or spina bifida, tends to be in a location similar to the prior pregnancy.
While not all the causes of anencephaly are known, many epidemiologic studies have demonstrated that folic acid supplementation prior to conception and during the first trimester can reduce the birth prevalence of spina bifida and anencephaly by at least 50%.
Many fetuses with anencephaly are stillborn or are spontaneously aborted, but more females are born with anencephaly than males.
Beginning in January 1998, the US Food and Drug Administration mandated that folic acid be added to all enriched cereal grain products. After this mandate, anencephaly affects about 1 in per 5,000 to 10,000 births in the U.S. Internationally, the number of births affected varies and is greatly influenced by the use of folic acid supplementation or food fortification, prenatal diagnosis, and pregnancy termination.
The U.S. Public Health Service advises women of childbearing age to take 0.4 mg of folic acid daily, either through supplements or fortified foods, even if those women do not think they are likely to become pregnant. This is recommended because the neural tube forms early, often before women realize they are pregnant.
Women are urged not to take more than 1.0 mg of folic acid daily unless advised by a physician because high doses of folic acid may mask the diagnosis of severe B12 deficiency (pernicious anemia).
Women who have had a previous pregnancy affected by a neural tube defect are advised to consume 4 mg of folic acid beginning 30 days before conception through the first trimester under the care of their physician.
Infants with anencephaly usually do not survive more than a few days or weeks. The role of healthcare providers is to provide a supportive environment that will enable the family to accept the diagnosis and make preparations for their loss.
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