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Vanishing twin syndrome, as its called, occurs when a twin dies in utero, usually as a fertilized egg early in a pregnancy. Many different things can happen afterward: the mother can miscarry, or the fetal tissue can be absorbed by the mother's body, the placenta, or even the remaining twin. When the latter happens, the surviving baby is born with two genetic signatures, making it what's known as a tetragametic chimera.

Related: Mysterious Mirror Twins

The consequences of this condition can seem quite strange, especially in a world where we rely on the accuracy of DNA testing. One famous case was that of Lydia Fairchild, who ran into trouble in 2002 when she was applying for public assistance. A maternity test determined that she wasn't mother of the children she had definitely given birth to. After a long court battle, the "real" mother was revealed to be her unborn twin, whose DNA resided in her body.

Related: 97% Of Your Genome Is Junk DNA

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