su·per·fe·ta·tion /-fiˈteʃən/
  異期複孕; 重孕
  su·per·fe·ta·tion /ˌsupɝfɪˈteʃən/ 名詞
  異期複孕,重孕,複孕
  Su·per·fe·ta·tion n.  Physiol. The formation of a fetus at the result of an impregnation occurring after another impregnation but before the birth of the offspring produced by it. This is possible only when there is a double uterus, or where menstruation persists up to the time of the second impregnation.
     In then became a superfetation upon, and not an ingredient in, the national character.   --Coleridge.
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  superfetation
       n : fertilization of a second ovum after a pregnancy has begun;
           results in two fetuses of different ages in the uterus at
           the same time; "superfetation is normal in some animal
           species"