Superfetation (also spelt superfoetation, based on a false etymology - see fetus) is the formation of a fetus while another fetus is already present in the uterus.

It is claimed to be common in some species of animals, but is extremely rare in humans. It can only occur where there are two uteri, or where the menstrual cycle continues through pregnancy. Animals which have been claimed to be subject to Superfetation include rodents (mice & rats) farm animals (horses & sheep) and primates (monkeys and man).

Reports of superfetation occurring long after the first impregnation have often been treated with suspicion and some have been clearly discredited. Other explantions have been given (and demonstrated) for different levels of development between twins. Artificially induced superfetation has, however, been demonstrated although only up to a short period after insemination. Superfetation has also been clearly demonstrated in poeciliidae fish [1].

External links

• Superfetation in Beef Cattle Cattle Doctoral Thesis; Joel Andrew Carter, Louisiana State University, Defended 2002-03-18
• superfetation in fish
• A case of twins with probable superfetation a report of a case of superfetation demonstrated through different paternity of twins.