superfoetation

from The Collaborative International Dictionary of English v.0.48

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

from 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.