Superfetation (also spelt superfoetation - see fetus) is the formation of a fetus while another embryo is already present in the uterus, from a different menstrual cycle. When there are two separate instances of fertilisation during the same cycle, 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 (kangaroos), and primates (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, 
In 2009, Todd and Julia Grovenburg of Fort Smith, Arkansas, received international media attention for Mrs. Grovenburg's conception of an additional child while already pregnant with a child conceived two and a half weeks earlier. If it were possible to carry both children to term, the birth of the first child would be expected in December 2009, whereas the second child would be due in January 2010.