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