Fetal acrania: five new cases and review of the literature.

Acrania is a lethal malformation in which there is an absence of the flat skull bones covering the brain. Five new cases are described, and a review of the English-language medical literature is presented. The sonographic differential diagnosis of acrania includes anencephaly, large cephalocele, osteogenesis imperfecta, and hypophosphatasia. The diagnosis of acrania can be established sonographically even in the first trimester if a large mass of disorganized brain tissue covered only by a thin membrane is detected.