Superfetation: An extremely rare situation in which a pregnant woman becomes pregnant a second time with another (younger) fetus. Superfetation is characterized by the fertilization and the implantation of a second oocyte in a uterus already containing the product of a previous conception. Superfetation is different from the process of twinning or multiple gestation and involves the conception of an additional fetus during an established pregnancy. With superfetation, the two fetuses have different gestational ages and due dates. Superfetation can occur in some animals but is so rare in humans that fewer than 10 cases have been reported in the medical literature.