A Woman Had Sex with Identical Twins and Now She Can’t Determine Who the Father Is

The judicial system doesn't yet have a clear solution

A British court recently waded into a case that sounds less like a legal proceeding and more like a daytime soap opera script.

A woman had sex with two identical twin brothers within the same four-day window. Nine months later, a child, known only as “P,” was born. Both men share the same DNA profile. Both men, according to every standard laboratory in the country, are the biological parent.

“Whilst DNA testing establishes that the child’s biological father is one of these twins, it is not possible to say which of them it is,” the documents — examined by PEOPLE and posted online by the United Kingdom’s Courts and Tribunals Judiciary — stated.

The Mirror Image Glitch in the Genetic Code

The mother (who wasn’t named for legal reasons) and one of the twins took the case to court. The other brother had originally been named as the father on the child’s birth certificate. The suing brother wanted to take legal responsibility for the child, who was born in 2018.

The case went to a family court judge, who declined to change the father on the birth certificate. The woman and the suing twin went to the Court of Appeal in London. But there too, a panel of judges ruled that it simply isn’t possible to know for sure who the father is.

Judge Madeleine Reardon previously found that “both brothers had had sex” with the woman “within four days of each other in the month when P was conceived”, and that it was “equally likely that each of the brothers is P’s father”.

Simply put, the court just couldn’t decide based on existing science.

How Paternity Tests Work

To understand why the court is stumped, you have to understand how we typically “prove” who someone is. Most paternity tests don’t read your entire genetic blueprint. Instead, labs look at Short Tandem Repeats (or STRs). These are specific spots in your DNA where the code repeats itself. Think of them like genetic bookmarks.

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In a normal case, you and a random stranger would have completely different STRs. Even if you’re comparing yours with a close relative, the difference would be clear. But identical twins are monozygotic. They started as a single fertilized egg that split into two. This means they share 100% of those genetic bookmarks. When the lab ran the tests for Child P, the results were equally consistent with Twin A and Twin B.

Identical twins aren’t perfectly identical. They start out with the same or nearly the same DNA sequence, but small difference can arive very early through mutations as cells divide. Over time, twins can differ through epigenetic changes (chemical changes that affect how genes are switched on and off). In a strictly scientific sense, identical twins are never truly and absolutely 100% identical.

So, while standard paternity tests can’t tell which identical twin is the father, advanced whole-genome or ultra-deep sequencing could find tiny mutations that one twin has and the other doesn’t. The catch is that this is specialized, expensive, and not always successful, so the answer can be yes in principle, but not reliably in every case.

The Implications Are Unsettling

Our legal system (and our society in general) is built on the concept of the individual. We’re all biological entities with a unique set of traits and responsibilities. But what happens when you can’t define that clearly?

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What happens to Child P?

The court did not identify a legal father, because it could not prove which identical twin was the biological father on the balance of probabilities. For now, the mother is the only clear holder of parental responsibility at present. The court also indicated that Child P should continue to have welfare-based relationships with both men through child-arrangements orders, rather than by treating either as the proven father.

The judge noted that by the time she reaches maturity, technology might be cheap enough to finally give her an answer. But for now, science just can’t reliably sort this one out.