Source : Webster's Revised Unabridged Dictionary (1913)

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.

         The female . . . is said to superfetate. --Grew.

Source : WordNet®

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