From The Collaborative International Dictionary of English v.0.48 [gcide]:
Superfetation \Su'per*fe*ta"tion\, noun [Cf. F. superf['e]tation.] (Physiol.) 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.
From WordNet (r) 3.0 (2006) [wn]:
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