The Collaborative International Dictionary of English v.0.48:

Superfetation \Su`per*fe*ta"tion\, n. [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.
   [1913 Webster]

         In then became a superfetation upon, and not an
         ingredient in, the national character.   --Coleridge.
   [1913 Webster]

WordNet (r) 3.0 (2006):

superfetation
    n 1: fertilization of a second ovum after a pregnancy has begun;
         results in two fetuses of different ages in the uterus at
         the same time; "superfetation is normal in some animal
         species"