The Collaborative International Dictionary of English v.0.48:

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

Bouvier's Law Dictionary, Revised 6th Ed (1856):

SUPERFOETATION, med. jur. The conception of a second embryo, during the
gestation of the first, or the conception of a child by a woman already
pregnant with another, during the time of such pregnancy.
     2. This doctrine, though doubted, seems to be established by numerous
cases. Beck's Med. Jur. 193; Cassan on Superfoetation; New York Medical
Repository; 1 Briand, Med. Leg. prem. partie, c. 3, art. 4; 1 Fodere, Med.
Leg. Sec. 299; Buffon, Hist. Nat. de l'Homme, Puberte.