65

Reviewed by Omotolani Ajileye

When we think of twins the word ‘two’ children born to the same parents.

What if a DNA test came back with a different outcome, that actually they are the result of two parents? This seems bizarre and impossible but it can and has happened. The ‘rare’ condition is known as Heteropaternal Superfecundation, when a woman has two eggs release in one menstrual cycle that both then become fertilised by different men.  Learn more about superfecundation from the Cleveland Clinic.

One of the cases that brought attention to this unusual biological possibility occurred in Brazil, where DNA testing revealed that a 19-year-old woman had given birth to twin boys who had different biological fathers.

The Brazilian Case That Surprised Doctors

The case occurred in Mineiros, in the Brazilian state of Goiás, and was reported by Brazilian media in September 2022.

According to reports from UOL, Folha de S.Paulo and Terra, the woman had sexual relations with two different men within a period of a few hours. She later became pregnant and gave birth to twin boys.

Initially, one man was believed to be the father of both children.

However, when the twins were around eight months of age, DNA paternity tests were ordered.

Unexpectedly, the tests revealed that the man fathered only one of the boys. The mother then remembered being involved with other sexual partners during the same time-span and DNA paternity tests were ordered for a second man. The tests indicated that this man was the biological father to the second child.

Physician Thilo Jorge Franco from Brazil stated that the case was a form of heteropaternal superfecundation.

Reportedly, the mother exclaimed she was shocked, and that she had no idea that twin children could possess fathers other than their conceived father.

So, How Can It Happen?

To understand the phenomenon, it helps to understand what happens during ovulation. The Mayo Clinic explains that ovulation occurs when an ovary releases an egg.

Normally, an ovary releases one egg during a menstrual cycle. Occasionally, however, two eggs can be released during the same cycle.

If both eggs are fertilised, the woman can become pregnant with fraternal twins. According to the American College of Obstetricians and Gynaecologists, fraternal twins develop when two separate eggs are fertilised by two separate sperm cells.

Interestingly, this happens when the sperm which fertilises the two eggs come from two different men. If a woman produces two eggs and has intercourse with two different men during her fertile window, one man’s sperm can fécondere one egg while the other man’s sperm fertilises the second egg. The outcome is that two babies developing in the same pregnancy would actually have two biological fathers.

Such phenomenon is known asheteropaternalsuperfecundation.

While sperm can live inside the female reproductive system for several days, the egg is only available for fertilisation during a shorter window (see the American Society for Reproductive Medicine – Ovulation and fertile window). This is why having sex with different men during a small timeframe around ovulation could potentially result in fertilisation of separate eggs by sperm from each man.

Why DNA Testing Is So Important

The Brazilian case demonstrates why the phenomenon can easily go unnoticed.

The twins may simply look like ordinary fraternal twins.

Fraternal twins already develop from two separate eggs and two separate sperm cells, so they do not have identical DNA. They share roughly the same amount of genetic material as ordinary siblings.

If both eggs are fertilised by the same father, the children are full siblings.

But if the eggs are fertilised by two different men, the children share the same mother while having different fathers.

On average, they therefore share about 25% of their segregating genetic material, similar to ordinary half-siblings. A forensic-science study published in the International Journal of Legal Medicine documented a heteropaternal twin case and described this genetic relationship.

Without DNA testing, there may be no obvious reason to suspect anything unusual. The National Human Genome Research Institute provides an overview of DNA and how genetic information is inherited.

How Rare Is It?

Heteropaternal superfecundation is considered extremely rare in humans.

A scientific case report published in 2020 stated that approximately 19 cases had been reported worldwide at that time.

When the Brazilian case was reported in 2022, the physician involved said he had identified about 20 records of the phenomenon worldwide and described it as exceptionally rare.

However, there is an important qualification: these figures represent documented cases, not necessarily every case that has ever occurred.

Researchers have pointed out that heteropaternal twins could go undetected if there is no reason to question paternity or conduct DNA testing. A 1992 study published in the journal Acta Geneticae Medicae et Gemellologiae found three cases in a database of 39,000 parentage records and estimated a higher frequency among dizygotic twins involved in paternity disputes—but the researchers warned that the finding should not simply be applied to the general population.

This means the phenomenon may be rare, but the number of documented cases should not be interpreted as an exact measure of how often it occurs.

Another Case Emerged in 2026

The phenomenon has continued to make headlines.

In 2026, British twin sisters Michelle and Lavinia Osbourne discovered through DNA testing that they also had different biological fathers.

The sisters, who were born in 1976, had spent most of their lives believing they shared the same father. DNA testing later revealed that each had a different biological father.

Their case was reported as the first documented case of heteropaternal superfecundation in the United Kingdom. The discovery was also featured in the BBC Radio 4 programme The Gift.

The British case demonstrates another important point: these cases can remain hidden for decades.

The twins had already grown into adults before DNA testing revealed the truth about their biological fathers.

Twins and Half-Siblings at the Same Time

So, are twins with different fathers actually twins?

Yes.

They are twins because they developed during the same pregnancy and were born from the same mother at the same time.

But genetically, they are also half-siblings because they do not share the same biological father.

They share their mother’s DNA but receive different paternal DNA.

That is what makes heteropaternal superfecundation such an unusual phenomenon.

What the DNA Tells Us

The stories from Brazil and the United Kingdom may sound extraordinary, but they demonstrate a simple fact about human genetics: biology does not always follow our assumptions about family relationships.

A woman can release two eggs during one reproductive cycle. If sperm from two different men fertilise those eggs within the relevant fertile window, the result can be twins who share a mother but have different biological fathers.

In the Brazilian case, it took a DNA test to uncover what had happened.

And years later, another DNA test revealed the same phenomenon in a pair of British twins.

So while twins having different fathers remains exceptionally rare, it is not a myth or a medical impossibility.

It is a documented biological phenomenon—and sometimes, DNA is the only thing capable of revealing it.

Read More:Parental Genetic Influence on Children’s Academic Performance Outcomes