Anencephaly is a relatively rare lethal birth defect that happens in about 1 in 10,000 pregnancies. It is caused by a failure of neural tube development (the fetal spinal cord) very early in pregnancy, and results in the absence of a major portion of the brain, skull, and scalp. This condition is incompatible with life, and infants with anencephaly are either stillborn or die shortly after birth.
In pregnancy, anencephaly is usually detected by ultrasound or by alpha feto-proten (AFP) serum screening. AFP is s protein made by the fetus and it crosses the placenta and is measured in mom's blood. When a fetus has anencephaly, an increased amount of AFP is seen in mom's bloodstream.