Noun
The formation of a fetus at the result of an impregnation occurring after another impregnation but before the birth of the offspring produced by it. This is possible only when there is a double uterus, or where menstruation persists up to the time of the second impregnation.
Source: Webster's dictionarySuperfetation can compensate for this loss by keeping embryos at various stages and sizes during development. Source: Internet
Superfetation is claimed to be common in some species of animals, but is extremely rare in humans. Source: Internet