What does "superfetation" mean?

Superfetation: 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.