superfetate

from WordNet (r) 3.0 (2006)

superfetate
    v 1: conceive when a fetus is already present in the uterus

from The Collaborative International Dictionary of English v.0.48

Superfetate \Su`per*fe"tate\, v. i. [L. superfetare; super
   above, over + fetare to bring forth.]
   To conceive after a prior conception, but before the birth of
   the offspring.
   [1913 Webster]

         The female . . . is said to superfetate. --Grew.
   [1913 Webster]