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]WordNet (r) 3.0 (2006):
superfetate v 1: conceive when a fetus is already present in the uterus