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. When there are two separate instances of fertilisation during the same cycle, rather than different cycles, it is known as superfecundation.
Superfetation is claimed to be common in some species of animals, but is extremely rare in humans. It can occur only 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 and rats), farm animals (horses and sheep), marsupials and primates (monkeys and humans). Superfetation has also been clearly demonstrated in poeciliid fish 
Reports of superfetation occurring long after the first impregnation have often been treated with suspicion and some have been clearly discredited. Other explanations 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.
In 2007, Ame and Lia Herrity, conceived 3 weeks apart, were born in the United Kingdom to Amelia Spence and George Herrity. In May 2007, Harriet and Thomas Mullineux, also conceived 3 weeks apart, were born in Benfleet, Essex UK to Charlotte and Matt Mullineux, 
Flavia d'Angelo, an Italian woman claiming to have become pregnant with triplets three months after initially getting pregnant, caused global media interest in 2001.  However, it was later revealed to be a hoax. 
- 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.