Numerous fathers called a genetic analysis center in Hanoi this week raising concerns about their newborn twins whom they doubted might not biologically be theirs, following a rare case of twins born of different fathers. 
 
 Nguyen Thi Nga, director at the Center for Genetic Analysis and Technologies (CGAT), said she received incessant phone calls from one father after another on Thursday morning, requesting consultancy on cases where their twins look nothing alike. 
 
 These calls came in the aftermath of the first recorded case of heteropaternal twins in northern Vietnam, covered by Tuoi Tre (Youth) newspaper on Wednesday. 
 
 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. The phenomenon is also known scientifically as heteropaternal superfecundation. 
 
 The twins in question were brought in for a DNA test at CGAT by their father prior to Tet, or Vietnam’s Lunar New Year festival, which fell on February 8. 
 
 According to Nga, the father in Hoa Binh Province was supposedly pressured by his relatives to take the test due to doubt about one of the twins’ real father, which had arisen from the baby’s appearance dissimilar from any family member’s. 
 
 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. 
 
 Though the names of the parties involved have been kept secret to protect their identity and privacy, the curious case has still made the news so far, sparking a wave of other concerned fathers across Vietnam who are in a similar situation. 
 
 No cause for concern 
 
 One of the men who rang Nga said he has a pair of twin daughters who will turn one next month. 
 
 The father had been troubled by the dissimilar appearances of his daughters since their birth, and his uneasiness has grown over time as their differences become more evident when they get older. 
 
 Throughout the conversation, Nga said, the concerned father kept reaffirming his unshaken faith in his wife, but confessed that some thought still burdened him and it was intensified after he heard about the reported case. 
 
 Nga said she had to keep reassuring the fathers who raised their ‘concerns’ that there was no cause for worry, as twins looking different from each other is not an uncommon phenomenon, and does not necessarily mean they are heteropaternal twins. 
 
 In fact, Nga added, fraternal twins, or twins who were fertilized by two different egg cells, are just like siblings who were born separately and can look very different from each other depending on whom they take after in the family. 
 
 “In these cases,” Nga said, “I always take it upon myself to advise the family not to take the DNA test, as heteropaternal twins are extremely rare in practice. Any doubt, no matter how small, can be devastating to their family ties, especially when such doubt hinges merely on the different appearances of the babies.” 
 
 There are around 10 pairs of heteropaternal twins recorded worldwide, who can have different skin colors or even completely different appearances from each another, Nga concluded. 
 
 Extremely unlikely, but not impossible 
 
 Answering a Tuoi Tre question on Thursday about the likelihood of similar occurrences, Vu Ba Quyet, director at the National Hospital of Obstetrics and Gynecology, said the probability is extremely low, but not completely impossible. 
 
 Quyet denied commenting on the credibility of recent reports of the pair of heteropaternal twins in Hoa Binh, but said it is possible for a female to bear twins from different males if she has engaged in sexual intercourse with the males within the same menstrual cycle, and her body has released two ova to be fertilized separately by their sperm. 
 
 The twins born this way can be different from each other in their appearance, genome, and gender, Quyet added. 
 
 Luu Thi Hong, a gynecologist and former general director at the Department of Maternal Health and Children under the Ministry of Health, said it is completely normal for fraternal twins to look different from each other. 
 
 According to Hong, the female body generally releases one ovum per month, though some women can release two at different times during a menstrual cycle, and the one that is churned out first, or either of the ova that comes out at the same time, is fertilized by the sperm. 
 
 “The conceptus takes seven to eight days to travel to the uterus and cling to its wall. Strictly speaking, a woman is considered pregnant only after the implantation of the conceptus has finished, so even if one of her ova has been conceived, the other ovum can still be fertilized within the seven to eight days it takes for the first conceptus to travel to the uterus,” Hong elaborated. 
 
 Like us on Facebook or follow us on Twitter to get the latest news about Vietnam! ||||| 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. 
 
 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. 
 
 Read More ||||| DNA testing has disclosed that a set of Vietnamese twins have two different fathers. There are only 10 such cases of bipaternal twins reported from across the world so far. — Photo dantri.com.vn 
 
 HA NOI (VNS) — A set of twins in the northern Hoa Binh Province were found to have different fathers, following DNA tests done by the Ha Noi-based Centre for Genetic Analysis and Technology. 
 
 This is the first case of its kind to be detected in Viet Nam. The two-year-old twins of the same sex look different, as one has thick and wavy hair, while the other has thin and straight hair. 
 
 The 34-year-old father, under pressure from a family member, agreed to a DNA test, which proved that he is the father of one infant. 
 
 In order to rule out possible mistakes by nurses at the hospital in giving them a baby of another couple, the wife also underwent a DNA test. The result shows that she is the mother of both children. 
 
 President of Viet Nam Genetics Association Le Dinh Luong said bipaternal twins were very rare. 
 
 There are reportedly about 10 sets of twins with different fathers across the world, including one in Poland, one in Spain, two in the United States and two in India. 
 
 He said the phenomenon had a scientific explanation. 
 
 Two eggs of the same mother get fertilised by two different fathers, within the same ovulation period. 
 
 When we see twins, we assume that both eggs were fertilised during a single act of intercourse. But it's quite possible for one egg to be fertilised during one act of intercourse, and the other during another. 
 
 It's only logical, then, that when a woman has sex with more than one man while she's fertile, "heteropaternal superfecundation" can occur. That is, each egg can be fertilised by a different father. 
 
 However, if the twins don't look alike, it should not be assumed that they have different fathers. Like siblings, twins with the same father can look completely different from each other. They may even appear to be of different races. — VNS
– 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.)