An intraventricular fetus-in-fetu, a malformed monochorionic diamniotic twin, was identified in a 1-year-old girl with motor delay and enlarged head circumference (Figure 1).
After surgical removal, whole-genome sequencing revealed identical single-nucleotide variants in the host child and fetus-in-fetu, with extensive de novo copy number gains in the fetus-in-fetu (Figure 2, eMethods, links.lww.com/WNL/C529), suggesting the significance of copy number variation during embryogenesis.
The intracranial fetus-in-fetu is proposed to arise from unseparated blastocysts. The conjoined parts develop into the forebrain of host fetus and envelop the other embryo during neural plate folding.1 Fetus-in-fetu can be distinguished from teratomas based on the younger age of presenting patients and the presence of vertebrae or internal organs.2
