A rare case of craniorachischisis associated with trisomy 18 is described. The diagnosis was made by endovaginal sonography during the first trimester of pregnancy. The case is interesting for two reasons. First, it confirms the importance of carrying out an accurate sonographic examination during the first trimester. Second, it shows us that the postmortem examination should be performed by an experienced anatomist: embryos need to be embedded in paraffin wax for microscopic observation and, at present, little is known about the abnormalities of embryo anatomy.