Superfetation (also spelt superfoetation, based on a hypercorrection — see fetus)'s the formation of a fetus while another fetus's already present in the uterus. When there're two separate instances of fertilisation during the same cycle, rather than different cycles, it's known as superfecundation. Superfetation's claimed to be common in some species of animals, but's extremely rare in humans. It can occur only where there're two uteri, or where the menstrual cycle continues through pregn… (More on Superfetation)