A panel of judges has ruled that it is impossible to determine which of two identical twins fathered a woman's baby.
The child (referred to as "P"), who was conceived in 2017, is now 8 years old, according to court documents.
The U.K. woman, who was only identified as "the mother," reportedly engaged in sexual relations with the identical twins just four days apart.
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She remained in a relationship with one twin, who was named on the birth certificate and initially acted as the legal father, the filing stated. When the relationship ended, it triggered a legal dispute.
The mother and the twin who is not listed on the birth certificate brought the case, seeking to overturn an earlier ruling and have him legally recognized as the child's father, the document detailed.
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The Court of Appeals ruled that legal paternity rights are only granted to a genetic father. While DNA testing shows a 50/50 chance for each twin to be the father, there is no definitive answer.
"Currently, the truth of P's paternity is that their father is one or other of these two identical twins, but it is not possible to say which," stated Lord Justice Moylan in the Court of Appeal ruling.
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Moylan added that the child's paternity "is binary and not a single man," highlighting the legal uncertainty created by the case.
Since neither twin can prove he is the biological father, neither can be given legal parental responsibility, the court declared.
The case is significant because it reveals a limit of DNA evidence, which is the foundation of many legal cases.
Identical twins have long presented a challenge in DNA testing, previous medical research has shown.
Monozygotic (identical) twins share virtually identical genetic profiles, including the markers used in standard paternity tests, meaning they "cannot be differentiated using standard DNA testing," according to a study published in Forensic Science International: Genetics.
There are more advanced methods, such as whole genome sequencing, that can detect some rare genetic mutations among twins, but researchers say these techniques are complex, expensive and not commonly used in legal cases.
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A study published in PLOS Genetics confirms that distinguishing between identical twins' DNA requires highly specialized analysis that goes beyond standard forensic testing.
