Neurosonoembryology by three-dimensional ultrasound.

High-resolution three-dimensional (3D) ultrasound has enabled the visualization of small embryos and fetuses, and embryology in vivo - '3D sonoembryology' - has been established based on conventional embryology. Recently developed imaging techniques allow the definition of in-vivo anatomy including visualization of the embryonic circulation and dynamic features that could not be characterized in fixed specimens. Three-dimensional ultrasound has facilitated increasingly accurate and objective prenatal diagnoses of cranium bifidum/spina bifida, holoprosencephaly and associated anomalies in the first trimester and may allow detection of pathologic central nervous system (CNS) development at an earlier gestational age. It may be no exaggeration to suggest that prenatal diagnoses of fetal abnormalities have shifted from second to first trimester. However, fetal brain develops rapidly in the second trimester, therefore early scanning covers only selected CNS anomalies described in this article and serial continuous observation in the second trimester will be required.