[Anencephaly at 20 weeks gestation. What is the optimal gestational age for early diagnosis?]

Anencephaly is a lethal defect resulting from a disturbed closure of the neural tube. Prenatal diagnosis during first-trimester-scan is essential but implies pitfalls. Three findings of anencephaly at 20 weeks gestation are reported. Fetuses with anencephaly are correctly identified at 12 to 13 weeks gestation. Anencephaly occurs in absence of the cranial vault. Ultrasound findings can be normal until onset of ossification has definitely failed. A first trimester scan at 12 to 13 weeks allows reliable diagnosis and active management of anencephaly. At this gestational age fetal anatomy can be screened in detail by transvaginal ultrasound and nuchal translucency can be measured.