It is called Situs Inversus and depending on where you look, they say it affects 1 in 7000-10000. When all of the organs are "flipped", also called mirror-image organs, then there aren't usually health problems associated with it. When only the heart is flipped, it seems to be linked to problems.
Some researchers at Harvard have been working on discovering why this happens and one of them suspects that drugs such as previcid during pregnancy may influence it. More than you needed to know!
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backwards heart 
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