Doctors
A rare case of bi-paternal twins emerges from Vietnam after stark differences in twin children's appearances leads to DNA testing.Getty Images

A stark difference in appearances of twins born to a couple in Vietnam led them to take the children for DNA testing, which confirmed the twins had different fathers. The unidentified twins were taken to a lab in Hanoi after the father's suspicions grew leading him to confirm whether or not he was the children's biological father.

"Our Centre for Genetic Analysis and Technology lab has tested and found a pair of bi-paternal twins," said Le Dinh Luong, president of the Genetic Association of Vietnam. "This is rare not only for Vietnam, but for the world." The DNA testing revealed that the man was the biological father of only one of the twins while his wife was found to be the biological mother of both of the children.

The twins born a few hours apart are now two years old and look nothing alike –while one has thick, wavy hair, the other twin has thin, straight hair. A hospital mix-up was ruled out after it was confirmed that the woman is the biological mother of both the twins, reported state-run Tuoi Tre newspaper.

Bi-paternal twins are born when a woman's eggs are fertilised by sperms from two different men during the same ovulation period. When both eggs are fertilised by the same man, normal fraternal twins are born, however when two different men are involved, the phenomenon is known as superfecundation.

Earlier in 2015, a US woman applying for child support from the father of her twin daughters was shocked after it was discovered that the man was the father of only one of the twins. Obstetrician-gynaecologist Jennifer Wu said that a sperm can remain viable for up to five days and ovulating women can produce over one egg, making it possible for a woman to be impregnated twice.