When Mia Washington gave birth to twin boys in 2010, something didn’t seem right. Although they were born just seven minutes apart, DNA testing revealed that they had different fathers.

Twins who are also half-siblings, also known as “superfecundation twins,” are rare, but can occur when a woman has sex with two men during the same ovulation cycle. In these cases, two eggs are fertilized by different sperm. Despite having different fathers, the children develop together as fraternal twins.

Most of the time, only one egg is released into the fallopian tube during ovulation. Sometimes, however, two to three eggs are released, especially when a woman is taking fertility hormones. These multiple eggs are what make fraternal twins (or triplets) a possibility — as well as the far rarer occurrence of having twins with different fathers.

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Doctors say that superfecundation twins are roughly one in a million, but in many cases, they may go unnoticed. After all, most people do not request paternity testing unless they have suspicions that a child is not theirs or are undergoing court proceedings.

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Mia and her fiance, James, weren’t splitting up or fighting over child support. Instead, they sought paternity testing after noticing that their twins had extremely different facial features. Although James is only the biological father of one of the boys, he’s vowed to raise both as his sons.

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