In an extremely rare occurrence, a mother in Vietnam has learned her fraternal twins have different fathers.
DNA testing for the two siblings at a clinic in Hanoi revealed that they are bi-paternal twins.
There are few details available about the children and their parents because of a confidentiality agreement between the family and the clinic, CNN reported Monday.
Local media in Vietnam, however, have said that the twins were born two years ago and look very different from one another. The DNA testing reportedly took place under pressure from family members.
Professor Le Dinh Luong, head of the agency that conducted the testing, told the BBC the results were "100 percent correct," albeit very unusual.
"This is rare not only for Vietnam, but for the world," he told Agence France Presse on Tuesday.
The scientific term for this highly uncommon occurrence is heteropaternal superfecundation. It occurs when a woman releases multiple eggs that are then fertilized by multiple men during a single ovulation period, meaning the children are conceived within a short time frame -- likely hours or days apart.
The case in Vietnam is believed to be the first of its kind recorded in the country, with less than 10 others documented worldwide.
The most recently reported instance of bi-paternal twins were sisters from New Jersey born in 2013. That case resulted in a paternity lawsuit.
According to a 1997 study published by genetics expert Dr. Karl-Hans Wurzinger, who testified in the 2013 court case, one in 13,000 reported paternity cases involved fraternal twins with different fathers.