Anencephaly is a neural tube defect in which the upper portion of the neural tube fails to close during the first few weeks of embryonic development, resulting in the absence of a major portion of the brain, skull, and scalp. On prenatal ultrasound, this condition can often be detected as early as the first trimester and is unmistakable by the second trimester, with the fetal head showing absence of the cranial vault above the orbits. It is one of the most common and severe neural tube defects and is unfortunately incompatible with life. Elevated maternal serum alpha-fetoprotein levels often prompt the ultrasound examination that leads to the diagnosis.
Sonography Term