The possibility of two eggs being fertilised during the same cycle by two different fathers, is one-in-a-million, doctors in China say.
A mum's affair has been exposed after a DNA test revealed her twins have different fathers, according to reports.
The anonymous woman from Xiamen, China, had requested a paternity test with her partner last year so that they could formally put their twin boys on the city's household register.
But they were in for a shock when the test results reportedly showed that one of the boys did not belong to the dad.
According to reports in the Fujian Daily newspaper, the woman's partner had long wondered why his twins did not look alike, and why one of them had different eyes, mouth and nose - and did not resemble him in any way.
However, he never ever considered the possibility that he was not the father of both, reports added.
The paternity test was carried out by the Fujian Zhengtai Judicial Authentication Center in Xiamen in East China's Fujian Province.
And understandably it caused a rift between the couple, with the wife allegedly accusing her husband of tampering with the results.
However, the mum-of-two eventually admitted that she had had a one-night stand with another man.
Image: What to expect.comNews of the rare case of super-fecundation - when two eggs are fertilised during the same cycle by two fathers - was made public by the Authentication Center's director, Zhang.
Zhang explained that super-fecundation means the mum had to have slept with her lover within hours or days of being first impregnated by her husband during the same fertility window.
She added: "This is a very special case. It means that the woman had sexual relations with two men - and was impregnated by them at the same time."
"They're fraternal twins. A very uncommon and rare case."
The director said the couple "sorted the matter out themselves".
The dad has since been quoted as saying: "I obviously still want my own child, but I'm unwilling to raise someone else's child."
