Fetus-in-fetu form of monozygotic twinning with retroperitoneal teratoma.

One of the commonest defects of blastogenesis is monozygotic twinning resulting in separate or conjoint twins, the latter including a category of internal, "endoparasitic," or fetus-in-fetu twins. We report the third known instance of endoparasitic twinning associated with a benign teratoma at age 1. The host, twin, and teratoma were monozygotic by DNA analysis. Copyright 2003 Wiley-Liss, Inc.