[First trimester sonographic diagnosis of acrania].

Anencephaly, a well-known lethal fetal malformation, was long considered to result from primary nonclosure of the neural tube. In the past few years other pathogenic mechanisms, such as reopening or degeneration of a closed neural tube have been suggested. High-resolution transvaginal ultrasonography, which provides fine visualization of the different stages in embryogenesis, allowed us to detect fetal acrania as early as the 12th week of gestation. Very high levels of alpha-fetoprotein, almost undetectable levels of unconjugated estriol (E3), and postabortion histology were consistent with anencephaly, suggesting that anencephaly is the end result of fetal acrania.