Twenty-nine healthy pregnant women were examined by transvaginal ultrasound to evaluate embryonic development in vivo between 7 and 12 weeks of gestation. The rhombencephalon with its fourth ventricle, the cerebellum and the choroid plexuses of the fourth ventricle were identified and measured. The cavity of the rhombencephalon, the future fourth ventricle, was always visible from 7 weeks, initially lying superiorly in the head of the embryo. The cerebellum and the choroid plexuses of the fourth ventricle became distinguishable during week 8. The volume of the rhombencephalic cavity was estimated. The shape and size of these rhombencephalic structures, their position in relation to each other and their relation to other brain structures changed specifically during the embryonic and early fetal period. This sonoembryological development corresponded to the descriptions in classical embryological literature.