superfetation

n. [ Cf. F. superfétation. ] (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. [ 1913 Webster ]

In then became a superfetation upon, and not an ingredient in, the national character. Coleridge. [ 1913 Webster ]