Superfetation (also spelled superfoetation – see fetus) is the simultaneous occurrence of more than one stage of developing offspring in the same animal.
[2] A 2008 French study found evidence to suggest that superfetation is a reality for humans, but that it is so rare that there have been fewer than 10 recorded cases in the world.
[4] In 2017, it was reported that an American woman who had agreed to act as a surrogate for a Chinese couple bore two babies, who were initially believed to be twins.
Before the adoptive parents could return home to China, however, it was discovered that one of the babies was, in fact, the biological son of the surrogate.
[11] Animals that have been claimed to be subject to superfetation include rodents (mice and rats), rabbits, horses, sheep, marsupials (kangaroos and sugar gliders), felines, and primates (humans).