无脑儿显示胎儿颅穹窿缺陷。作为神经管缺陷的一种类型，无脑儿的发病率过去被报道为1～5/1000。然而,由母亲妊娠期前或妊娠期的叶酸饮食供应能减少此种先天缺陷。目前在一些地方新生无脑儿发病率低于1/10000。
The absence of the fetal cranial vault in anencephaly is shown here. The incidence of anencephaly, one form of neural tube defect, has been reported to occur as frequently as 1 to 5 in 1000 live births in the past. However, dietary folic acid supplementation by mothers-to-be before and during pregnancy can reduce the incidence of such birth defects. The rate of anencephaly is now less than 1 in 10,000 live births in some places.