superimpregnation --> superfetation
(Science: physiology) The formation of a foetus 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)
Origin: Cf. F. Superfetation.