无脑儿是胎儿颅穹窿缺陷。暴露在羊水内脑组织不能得到发育。无脑儿是神经管缺陷的一种类型，是单纯性先天性缺陷，与染色体异常无关。
Anencephaly is absence of the fetal cranial vault. Exposure of cerebral tissue to amniotic fluid precludes brain development. Anencephaly is a form of neural tube defect that is typically an isolated birth defect that is not related to chromosomal abnormalities.