A

Absence of the cranial vault (acrania-exencephaly sequence) can be recognized on 11-14-week ultrasound, allowing reliable first-trimester diagnosis of anencephaly, whereas encephalocele, meningocele and microcephaly are typically confirmed later in gestation when skull and brain measurements are better defined [Chatzipapas 1999, PMID 10204212; Sen 2001, PMID 11447925].