A peculiar subtype of holoprosencephaly, middle interhemispheric variant (MIH), which is characterized by a partial posterior interhemispheric fusion of the brain, has been described in children. We describe the features of a case of a possible new MIH at 26 weeks of gestation, diagnosed using prenatal sonography and magnetic resonance imaging and confirmed by postmortem examination. This malformation of the brain was associated with an unusual appearance of the corpus callosum and rare chromosomal abnormality: a 45X/46,XX/47,XX,+ 18 mosaicism. Copyright © 2006 ISUOG. Published by John Wiley & Sons, Ltd.