Ovophagy

Ovophagy is a rare though widely observed condition in the reproduction of certain animals, whereby fetuses will devour each other prior to birth. The condition was most noticeably ascribed to the Bigeye thresher shark. Subsequent research has shown ovophagy to be sporadic and highly uncommon among the vast majority of animal species.

Ovophagy is morphologically not possible among human fetuses; that is, it is physically impossible for one human fetus to devour or otherwise "absorb" a sibling. However, in fiction, Stephen King's The Dark Half centers on a case of ovophagy among twin boys.