When relatives insisted his twins did not look alike, the 34-year-old man from northern Hòa Bình province in Vietnam took the infants for DNA tests. According to the online newspaper Dan Tri, one twin had thick wavy hair, while the other had thin and straight hair.

What the tests revealed marked a rare victory against the odds in reproductive biology – though that was perhaps not the first thought to cross the parents’ minds. The newborn twins, according to the Centre for Genetic Analysis and Technology lab in Hanoi, had two different fathers.

The man from Hòa Bình was the father of only one of the twins. The Y chromosome from the other infant did not match his own.

Le Dinh Luong, the president of the Genetic Association of Vietnam, said his lab had tested and found a pair of bi-paternal twins, adding: “This is rare not only for Vietnam, but for the world.” The twins, who are now two, were born on the same day and are the same sex.

DNA testing of the mother to rule out a mix-up at the hospital dismissed that possibility, and confirmed that both children were hers, the newspaper said.



In the world of embryology, the birth of twins to two different fathers is not unheard of. It requires the woman to ovulate two eggs at the same time, and have viable sperm from two different men waiting inside her to fertilise the eggs.



A 1992 report on paternity suits over non-identical twins – those produced when two separate eggs are fertilised and carried at the same time – found that 2.4% had different biological fathers. It is common enough to earn its own lengthy name: heteropaternal superfecundation.



“For this to happen there’s got to be whole series of events that line up and happen in same few hours. Not only has she got to have sex with two men in a relatively short time, that has got happen at a time when she’s ovulating two eggs rather than the usual one,” said Allan Pacey, a professor of andrology at the University of Sheffield. “The trick is that the woman would have to have intercourse with the two men within five or six hours of each other.”

Though rare, or rarely reported, in humans, heteropaternal superfecundation is common in the animal world. Domestic cats and dogs have twin offspring from different fathers, as do birds and other animals.



While the fallow period between the two acts could, in theory, have been longer – sperm can last for several days in a woman – the odds favour a brief period of mere hours, Pacey said. “If the first man had sex with her five days before ovulating and the second man had sex one day before ovulation, the first man’s sperm will be dying off and less likely to fertilise one of the eggs. To maximise chances of it happening, it has to be really close.”

Dagan Wells, a fertility specialist at Oxford University, said the time window during which the double fertilisation occurred could be longer than Pacey suggests, because the woman may have ovulated two eggs over 24 hours.