A woman who became pregnant after having sex with two identical twins has been told that it may be impossible to ever determine which brother is the biological father of her child.

The case, believed to be the first of its kind in the UK, has now reached the Court of Appeal in London. The mother has brought the case forward alongside one of the twins, as they seek legal recognition of parental responsibility for the child.

The situation dates back to 2017, when both brothers separately entered into a casual relationship with the same woman, reportedly without knowing about each other. During a four-day period in which she is believed to have conceived, she had sex with both men.

She later gave birth to a baby girl in 2018, who is now eight years old. The question of who her biological father is remains unanswered, and may stay that way.

DNA paternity testing, typically the definitive method in such cases, failed to provide clarity, according to Tyla. Because the men are identical twins, both tests returned positive results, leaving a 50/50 chance that either could be the father.

Initially, one of the brothers was named as the father on the child’s birth certificate. However, the other twin challenged this, leading to a prolonged legal battle.

“They have both claimed her”

A panel of judges has now ruled that it is “not possible” to determine which twin is the biological father. As a result, the brother already listed on the birth certificate will remain there. However, his parental responsibility has been temporarily removed while the courts continue to examine the case.

In a judgment issued earlier this month, Sir Andrew McFarlane acknowledged that science may one day provide an answer, but not without significant difficulty.

“It is possible, indeed likely, that by the time [the child] reaches maturity it may be possible for science to identify one father and exclude the other twin, but, for the coming time that cannot be done without very significant cost, and so her ‘truth’ is binary and not a single man.”

He added: “The failure to prove a fact means that that fact is not proved, it does not mean that the contrary is proved.

“There is a distinction between something being not proven, and making a positive declaration that the fact asserted is not true.”

The case follows an earlier family court hearing in 2024, where a judge was also unable to determine which twin was more likely to be the father.

“They have both claimed her; and they are both pursuing this claim at considerable financial and personal cost, including a cost to their own relationship which, before this issue arose, was close.”