It might sound like a wayward twist from a movie, but it is possible for fraternal (not identical) twins to come from two different dads. The circumstances must be very specific, but some experts estimate that 1-2 percent of fraternal twins have different fathers.
In May 2015, The New Jersey Law Journal and The New York Times reported an unusual case in Passaic County, New Jersey. A mother of twins applied for child support, claiming that her former partner was responsible for both of her children. In testimony, however, she admitted to a second relationship with a different, unidentified man. She had this relationship within a week of her relations with her first partner. The court ordered a paternity test, which found that her former partner was responsible for only one of the twins. It was a landmark decision.
But how is this scientifically possible? It's a phenomenon known as superfecundation, and it occurs when two different eggs are fertilized during the same menstrual cycle. In this case, the mother had a sexual relationship with one man, ovulated, and then had the other relationship with the unidentified second man. One man's sperm fertilized one egg, and the second fertilized the other egg.
Superfecundation is also more common thanks to assistive reproductive technology - if, for example, a male gay couple both contribute sperm to a pregnancy.
According to Karl-Hans Wurzinger, who testified in the Passaic County case and runs the Identity Testing Division at Laboratory Corporation of America, about one in 13,000 reported paternity cases involve twins with separate fathers.