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. Heteropaternal twins are born when two ova (egg cells) from the same menstrual cycle are fertilized by sperm from separate acts of sexual intercourse with two different males. It's only logical, then, that when a woman has sex with more than one man while she's fertile, "heteropaternal superfecundation" can occur. The results came as a shock to the said dad that he is the biological father of only one of the two babies, while his wife mothers both.  Story highlights Family has DNA testing after relatives note twins look very different from one another 
 
 Twins who have different fathers are extremely rare, scientists say 
 
 (CNN) Scientists in Vietnam have confirmed a case of bi-paternal twins, or twins who have different fathers.
– Extended family butting in with opinions about your kids is nothing new—but this case in Vietnam is a wild exception. CNN reports that a Vietnamese couple had their 2-year-old twins genetically tested after family members kept harping about how different the children looked. What the Center for Genetic Analysis and Technologies in Hanoi discovered: The twins are bi-paternal, meaning they share only a mother. The revelations about the family pretty much end there. The twins' gender was not revealed, and the only identifying details given by Vietnam's state-run news agency VNS is that the children have very different hair (thick and wavy vs. thin and straight) and live in Hoa Binh province with their 34-year-old father and mother, whose age wasn't given. Le Dinh Luong, president of the Genetic Association of Vietnam, says he's unaware of any case like this in his country. That hasn't stopped paranoid fathers from phoning the Hanoi center over fears about their twins, reports the Tuoi Tre paper. It's rare, but certainly not unheard of in the US. Last year, a New Jersey judge ruled a man is father to one twin while another man is father to the other, in a case that had child-support implications. The judge in that case cited two other similar court cases, and a 1997 article by identity testing expert Karl-Hans Wurzinger, who testified in the case, put the number of reported paternity cases involving superfecundation (the fathering of fraternal twins by two males) at one in every 13,000. CNN reports that the lifespan of the egg and sperm allow for roughly a week-long period in which a woman could become pregnant through two acts of intercourse. (This man's son was "fathered" by his unborn twin.)