A defect of neural tube
development, anencephaly occurs in approximately 1 in 1000 births (see note) and if the mother has had a child with a neural tube defect, her chances of a second are 1 in 50 (after two children, another is 1 in 25). The neural tube is a portion of the developing embryo
that eventually forms the brain
and spinal cord
. The tube is supposed to fold and close between the third and fourth week of development. Anencephaly occurs when the tube does not close (the "cephalic
" or head end) and development of the brain is seriously affected. The infant does not develop a forebrain
. Often there is a lack of scalp and skull covering ("calvarium") for what brain tissue that does develop, which is usually just a basic brain stem
Most infants are either stillborn or die within days as there is no treatment for this defect. Ones that are born alive will usually not be able to see or hear, will not be able to feel pain, and will be unconscious (with no chance of ever becoming conscious). Since there sometimes is a rudimentary brain stem, some basic reflex actions may be possible (breathing and possibly aural and tactile responses).
Certain abnormal malformations are often present in infants with anencephaly (besides the lack of upper skull formation): cleft lip and palate, eyes that abnormally bulge out, and sometimes abnormalities in the hands and feet.
While there are tests that diagnose the condition in the developing embryo, there is no determined cause. It doesn't appear to genetically related, though as noted above, prior pregnancies of this kind tend to increase chances of reoccurrence, suggesting a possible factor. Many believe that environmental factors (probably toxins and pollutants) may play an important role in anencephaly as well as the mother's diet. It has been related to low folic acid levels and hypervitaminosis A ("vitamin A toxicity," an excessive accumulation of the vitamin in the system).
Another condition resulting from a neural tube defect is Spina bifida.
From various sources. Particularly helpful was the fact sheet from The National Institute of Neurological Disorders and Stroke/National Institutes of Health.
Note: it is difficult to determine the actual incidence of these births since it is believed that many of the infants spontaneously abort.