What sort of problem is anencephaly?

Failure of closure. The fetus is busy around 20-30 days in when the nervous system starts forming along a line like a trench. The trench folds in on itself and begins to form a tube, sealing itself in the middle first and moving toward both ends like two zippers. When the top end fails to close, many brain structures don't form & we have anencephaly.At the bottom, we get spina bifida.
Birth defect. Failure of the brain to develop normally in the fetus.