RT Book, Section A1 O'Rahilly, Ronan A1 Müller, Fabiola A2 Timor-Tritsch, Ilan E. A2 Monteagudo, Ana A2 Pilu, Gianluigi A2 Malinger, Gustavo SR Print(0) ID 1138378209 T1 PRENATAL DEVELOPMENT OF THE BRAIN T2 Ultrasonography of the Prenatal Brain, 3e YR 2017 FD 2017 PB McGraw-Hill Education PP New York, NY SN 9780071613064 LK obgyn.mhmedical.com/content.aspx?aid=1138378209 RD 2024/11/10 AB KEY POINTSThe embryonic period (the first 8 postfertilizational weeks) is subdivided into 23 morphological stages, which, because they are based on internal as well as external criteria, cannot be identified with confidence by ultrasonography.The three major divisions of the brain are found early (stage 9), closure of the neuropores seals the cerebrospinal cavity at 4½ weeks (stage 13), and the five main subdivisions of the brain are visible at 5 weeks (stage 15).The telencephalon is identifiable already at 4 weeks (stage 10) and begins to diverticulate at 5 weeks (during stage 14), but holoprosencephaly is more than a mere failure of diverticulation and may begin as early as 3 weeks (stage 8), as can cyclopia and anencephaly.The appearance of the cortical plate (stage 21) heralds the beginning of lamination of the cerebral cortex, the basal nuclei and internal capsule are progressing, and the brain is developmentally advanced at the end of the embryonic period.Prominent features during the fetal period are the C-shaped structures, including the corpus callosum, and the appearance of sulci and gyri on the cortical surface at the middle of prenatal life.