This article is from the Pregnancy Screening FAQ, by Lynn Gazis-Sax (email@example.com) with numerous contributions by others.
2.c The two main defects are spina bifida (open spine) and anencephaly.
The problems associated with spina bifida will vary with the size, location
and nature of the opening in the spine. Problems range from mild backaches
to severe mental retardation, paralysis, bowel and bladder control problems
and leg deformities. Generally speaking, the AFP test is more likely to
detect the more severe forms of spina bifida.
Anencephaly is a failure of the brain and skull to fully develop. It is
incompatible with life.