A pair of twins born in Vietnam, one with thin, straight hair and the other with thick, wavy tresses, are a sight to behold. However, their DNA at the Center for Genetic Analysis and Technology in Hanoi has scientists baffled, as it confirms that they have different fathers.

Le Dinh Luong, president of the Genetic Association of Vietnam, told CNN that a couple in Vietnam brought their twin sons in for a test after the couple's family noted the obvious difference in the appearance of the two.

"Our Center for Genetic Analysis and Technology lab has tested and found a pair of bi-paternal twins," he said. "This is rare not only for Vietnam, but for the world."

When a woman has intercourse with two different men within the same window of ovulation, it can lead to a case of bi-paternal twins, also known hetero-paternal superfecundation.

Hilda Hutcherson, a clinical professor for obstetrics and gynecology at Columbia University, said that cases of fraternal twins happen when a woman discharges two eggs instead of one during her ovulation and both the eggs are fertilized. In the case of bi-paternal twins, each egg is fertilized by a sperm of different man. She added that an egg lives for 24 hours in a woman's body but a sperm can live up to five days. So it is possible that if she has sex with one man on one day and then another man on the other day, these two sperms inside her could fertilize each egg.

Luong said the results were "100% correct" and referred to the situation as "an extremely rare case."

"There are less than 10 known cases of twins with different fathers in the world," he said. "There might be other cases but the parents and/or the twins were not aware of it or didn't want to announce it."

The Vietnamese couple was pressured by the family to undergo a DNA test to ensure that the twins were not mixed up in the hospital. However, the results revealed a different story. While the mother's DNA matches that of the twins, the father's didn't. After the test, a possible hospital mix-up was ruled out, as the wife was confirmed as their biological mother.