A Vietnamese couple recently brought their fraternal twins to the Center for Genetic Analysis and Technologies, in the country's capital of Hanoi, to have DNA tests, after pressure from extended family members who noticed the children did not look alike, said Le Dinh Luong, president of the Genetic Association of Vietnam.
The tests showed there was no mix-up in the hospital: The twins have the same mother, but different fathers.
Luong said the family was surprised by the revelation, and looking for the best way to deal with the situation.
He declined to give more details, because of a confidentiality agreement with the parents.
VNS, Vietnam's state-run news agency, reported the family is from northern Hoa Binh province and the children are 2 years old. One sibling has thick and wavy hair, while the other has thin and straight hair, VNS said.
Twins who have different fathers are extremely rare, Luong said, adding this is the first case in Vietnam that he knows of.
Another case was reported in the United States last year, when a court ordered a man from New Jersey to pay child support for one girl in a set of twins, after genetic tests showed he was not the father of the other.
A woman's egg has a life span of 12 to 48 hours, and a sperm is viable for seven to 10 days, so there's about a week's time for potential overlap and the fertilization of two eggs by two sperm from separate acts of sexual intercourse, Dr. Keith Eddleman, director of obstetrics at Mount Sinai Hospital in New York told CNN last year.