AbstractMurine trisomy 16 (mts16) placentas and fetuses, 17‐day gestation age, were examined histologically and by magnetic resonance imaging at 9.4 T and compared with control littermate tissues. Placentas were studied by immunohistochemical methods, at 15‐days gestational age, for expression of the major histocompatibility complex (MHC) class I H‐2Kkcell surface marker. Immunohistochemical studies revealed a markedly increased expression of the MHC marker H‐2Kkon cells in the labyrinth of the placenta of mts16. There were differences between the magnetic resonance (MR) images of the trisomic and normal placentas, which may be correlated with the increased expression of H‐2Kkin the mts16 placental labyrinth. The decidual and labyrinthine components of the normal placentas showed similar high signal intensities (SI) while in trisomic placentas a marked high SI was characteristic only of the decidual region on proton spin density images. The MRI also revealed a smaller cerebellum in the ts16 fetuses. The potential effects of the compromised structure of the placental labyrinth and the overexpression of the H‐2Kkmarker on the mts16 neural and placental dysgenesis ar