| Sumario: | Objective: Rhombencephalon, which is responsible for the development of the cerebellum, cerebellar vermis and cisterna magna, comprises the posterior part of the three brain sacs in the embryo. In this study, we investigated whether the size differences of the rhombencephalon in the first trimester had any influence on the development of the posterior fossa structures. Material and Methods: In 87 healthy embryos, rhombencephalon was evaluated using high frequency (7.5 MHz) probe through the transabdominal ultrasonography between 7-10th weeks of pregnancy. Rhombencephalon was measured in sagittal planes and the mean diameter was calculated. In addition, the subjects were re-evaluated ultrasonographically in the second trimester in order to detect the development of the cerebellum, cerebellar vermis and cisterna magna, which are formed from the rhombencephalon between 18-24th weeks of pregnancy. Results: Between the 7th and 10th gestational weeks, the minimum, maximum and mean rhombencephalon diameters were 1.8 mm, 4.5 mm and 2.9 mm, respectively. Conclusion: The size of the rhombencephalon may vary within the same week or from week to week. In the present study, no adverse effect of these differences was established in size on posterior fossa structures.
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