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Holoprosencephaly (HPE) is a complex human brain abnormality caused by incomplete cleavage of the prosencephalon into the right and left hemispheres, occurring between the 18 th and 28 th day of gestation. Different levels of increasing severity are defined: (1) lobar HPE, where the right and left ventricles are separated, but with some continuity across the frontal cortex; (2) semilobar HPE with a partial separation; and (3) alobar HPE, the most severe form, with a single brain ventricle and no interhemispheric fissure. Mettler [1] described the dorsal cyst of HPE as a structure that “occupies the area of the calvarium above the dorsocaudal aspect of the diencephalons” and indicated that the “walls of this cyst are always directly continuous with the most caudal parts of the walls of the telencephalon medium and its cavity communicates directly with the common ventricle of the telencephalon”. Furthermore, he added that a variable amount of the anterior wall of the dorsal cyst is “fused with the dorsal surface of the thalamus, sealing over the third ventricle”. Other authors have suggested that aqueduct stenosis or atresia results in dorsal cyst formation [2,3]. There have been many reports on the nature of dorsal cyst in HPE, but it was always noted after birth. HPE is the most common but lethal defect in the congenital cerebral abnormalities. It is a heterogeneous etiologic disease that can be caused by both a teratogenic and/or a genetic basis. Alobar or semilobar HPE is usually combined with a facial defect due to the missed embryogenesis, but early prenatal diagnosis of semilobar HPE with a dorsal cyst and no facial defect is rare. Herein, we report a case of early detection of HPE with a dorsal cyst and no facial defect in a low-risk mother, to present the distinctive features of semilobar HPE and the hypotheses involving a dorsal cyst. A 27-year-old, gravida 1, para 0, woman first visited our prenatal clinic at 9 weeks of gestation. Previous obstetric history and family history were unremarkable. An ultrasound (US) scan confirmed an intrauterine pregnancy with a fetus of crown–rump length of 2.5 cm without abnormal findings. During the second visit at 12 weeks’ gestation, a second US scan illustrated a nuchal translucency measuring 1.8 mm and a clear nasal bone structure. However, a dorsal cyst over the magnum cisterna in the transverse view (Figure A) was noted. The follow-up scans showed abnormalities of the forebrain, including partial absence of the midline echo, partial fusion of the thalami and abnormal ventricular configuration (Figures B and C), but facial structure was normal. Semilobar type of HPE with a dorsal cyst was diagnosed at that time. The other organs and systems, including cardiovascular, gastrointestinal and urinary systems, were unremarkable under the sonographic examination. Examinations for the presence of toxoplasmosis, rubella, cytomegalovirus, herpes (TORCH) were performed but without positive findings. Consequently, amniocentesis was scheduled for the next visit.