Superfetation (also spelt superfoetation - see fetus) is the simultaneous occurrence of more than one stage of developing embryo in the same animal. In mammals it manifests as the formation of a fetus from a different menstrual cycle while another embryo is already present in the uterus. 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. In mammals it can occur only where there are two uteri, or where the menstrual cycle continues through pregnancy. The risk with superfetation is that the second baby is often born prematurely, which can increase its odds of experiencing lung development problems.
Animals that have been claimed to be subject to superfetation include rodents (mice and rats), farm animals (horses and sheep), marsupials (kangaroos and sugar gliders), 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 1992, Taylor and Evan Barth, conceived 4 weeks apart, were born in Hawaii, USA to Michelle and John Barth. Taylor, the one conceived later, was born first.
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. Grovenburg's obstetrician reported that cases of superfetation "can only be confirmed after delivery by chromosomal and metabolic studies on the baby."
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