A DNA test has revealed that a pair of fraternal twins in Vietnam were fathered by two different men.
According to local reports, the twins were tested after their family noticed they looked different.
This is not the first time such an occurrence has been reported, but it is very rare, with only a few publicly known cases.
Called heteropaternal superfecundation, it happens when a woman's eggs are fertilized by two men within a short period of time.
Professor Le Dinh Luong, president of the Hanoi-based Vietnam Genetic Association which did the DNA testing, told the BBC that the results were "100% correct."
"There are only less than 10 known cases of twins with different fathers in the world. There might be other cases but the parents and/or the twins were not aware of it or didn't want to announce it," he told the news outlet.
The last publicly reported case of heteropaternal superfecundation was last year involving twin girls in New Jersey. Their paternity came to light through DNA tests after the mother went to court seeking child support. The judge said he could find only two other such cases on record in the U.S.
But experts say these occurrences may be more common than we think.
Dr. Cynthia Austin, a fertility specialist at Cleveland Clinic Women's Health Institute, explained the mechanics of how heteropaternal superfecundation happens.
"Normally, when a woman ovulates, she's only producing one egg, but that's not always the case," she told CBS News. "Sometimes there are two." When two sperm fertilize two eggs, fraternal twins are formed. However, since sperm can remain alive for several days inside the body, it is possible that sperm from two different men could fertilize two eggs during the same menstrual cycle. As a woman is fertile for five to seven days a cycle, "the intercourse with different partners can happen days apart," Austin said.
"Sometimes it's very obvious, if the father is one race and the babies are two different races," Austin said. However, there may be instances where both the mother and father are unaware that such an incident has occurred. "I would say the vast majority of times, twins with different fathers, it goes unnoticed," she said.