Twins with different fathers: the science behind heteropaternal superfecundation

Most people assume that twins – whether identical or non-identical – share the same biological father.

And in most cases, that assumption is correct. But not always.

A recent documentary on BBC Radio 4 in the UK shattered this idea in the most remarkable way with the story of Michelle and Lavinia, non-identical twins who discovered through an at-home DNA test that they do not share the same biological father.

Born to the same mother, minutes apart, they are in fact genetically half-sisters.

Their story is the result of an extraordinarily rare biological phenomenon called heteropaternal superfecundation. So rare, that only around 20 cases have ever been identified worldwide.

Michelle and Lavinia are also believed to be the only twins with different fathers ever formally documented in the UK.

So how is it possible? And what can modern DNA science reveal about twin relationships and family connections that might otherwise remain hidden?

What are twins?

To understand how twins can have different fathers, it helps to understand how twins are formed, and what sets the different types apart.

The two most common types of twins are identical and non-identical twins.

Identical (monozygotic) twins

Identical twins develop when a single fertilised egg spontaneously splits into two embryos shortly after conception.

Because both embryos originate from the same egg and sperm, identical twins share virtually 100% of their DNA. They are always the same sex and are often strikingly similar in appearance, though subtle differences can emerge over time as a result of environmental factors.

Non-identical (fraternal or dizygotic) twins

Non-identical twins develop when two separate eggs are released during the same menstrual cycle and fertilised by two separate sperm cells.

Because they originate from two entirely different eggs and sperm, non-identical twins share around 50% of their DNA, the same proportion as any two siblings born at different times.

They can be the same sex or different sexes and may look no more alike than any other brother or sister.

It is this type of twin – where two eggs are released and fertilised separately – that makes heteropaternal superfecundation biologically possible.

Other rare types of twins

Most people are familiar with identical and non-identical twins.

However, human biology is far more complex and varied than many people release, and there are actually several rare and fascinating types of twins that exist.

• Semi-identical (sesquizygotic) twins, where an egg is fertilised by two sperm before dividing. This results in twins who share 100% of their mother’s DNA but different proportions of their father’s, making them genetically more similar than fraternal twins but not completely identical. They may even be different sexes.
• Mirror image twins, a subset of identical twins who develop with reversed features. For example, one may be left-handed where the other is right-handed, or birthmarks may appear on opposite sides of the body. This occurs when the embryo splits slightly later than usual, around 9 days after conception.
• Polar body (half-identical) twins, a theoretical concept where an egg splits before fertilisation and each half is fertilised by a different sperm. These twins would share around 75% of their DNA. Clinical evidence in humans remains extremely limited.
• MoMo twins (monochorionic monoamniotic), one of the rarest and highest-risk types of identical twins, who share both a placenta and an amniotic sac. This carries significant risks during pregnancy, including twin-to-twin transfusion syndrome and cord entanglement, requiring close monitoring.
• Superfetation twins, where a woman conceives a second time while already pregnant, resulting in twins born at different stages of development. This is incredibly rare in humans.
• Chimeric twins, where two embryos fuse very early in development to become one individual with two distinct sets of DNA. Chimerism often goes undetected unless DNA testing reveals unexpected results, or there are visible signs such as different eye colours or skin tones.
• Conjoined twins, where the embryo splits very late and does not fully separate, resulting in twins whose bodies remain physically connected at birth.

These unusual types of twins highlight just how complex and varied human biology can be.

An AlphaBiolabs’ Twin DNA Test can determine whether twins are identical, non-identical, or potentially semi-identical.

Learn more: Identical vs fraternal twins
Learn more: Fun facts about twins

What is heteropaternal superfecundation?

Heteropaternal superfecundation is the biological process by which non-identical twins are conceived with two different biological fathers.

For it to occur, several conditions must be met simultaneously:

• A woman must release more than one egg during the same menstrual cycle
• Each egg must be fertilised by sperm from a different man
• Both fertilised eggs must successfully implant and survive to term

The fertilisation of both eggs typically occurs within the ovulation window of 12 to 24 hours.

However, because sperm can survive inside the uterus for several days after intercourse, it is theoretically possible, though extremely unlikely, for heteropaternal superfecundation to result from encounters that are a number of days apart.

Because it requires two eggs to be fertilised, heteropaternal superfecundation can only ever occur in non-identical (fraternal) twin pregnancies. Identical twins, who develop from a single fertilised egg that subsequently splits, are not affected by this phenomenon.

Heteropaternal superfecundation is considered extremely rare in humans, with only around 20 cases ever identified worldwide. As illustrated by the story of Michelle and Lavinia, its true prevalence is difficult to determine, partly because twins are not routinely subjected to DNA testing that would reveal different paternity.

Factors that may increase the likelihood of heteropaternal superfecundation include a tendency towards hyper-ovulation (releasing multiple eggs per cycle) and sexual activity with more than one partner within a short timeframe.

Learn more: Can twins have different fathers?

What can DNA testing reveal about twin relationships?

The story of Michelle and Lavinia is a striking example of how at-home DNA testing can surface truths about biological relationships that might otherwise remain hidden for a lifetime.

In their case, a home DNA test revealed not only that they did not share the same biological father but set in motion a journey that ultimately led both women to discover their respective fathers’ identities, each with very different outcomes.

For twins specifically, DNA testing can answer questions that even a lifetime of shared experience cannot.

Some of the most common reasons twins or their families seek DNA testing include:

• Confirming zygosity: determining whether twins are identical, non-identical, or semi-identical. This is not always clear from appearance alone, and in some cases is not even conclusive from ultrasound during pregnancy.
• Medical reasons: where one twin has been diagnosed with a hereditary condition, knowing whether siblings are genetically identical can be important for assessing risk in the other twin.
• Curiosity: many twins and their families simply want a definitive scientific answer, particularly where there has been long-standing uncertainty or debate
• Unexpected results from other DNA tests: as in Michelle and Lavinia’s case, results from ancestry or other home DNA tests can raise questions about biological relationships that require further investigation.

AlphaBiolabs’ Twin (Zygosity) DNA Test

For twins and their families seeking a definitive answer about zygosity, AlphaBiolabs offers the best Twin DNA Test available in Ireland.

What makes our Twin DNA Test the best available.

• €119 + Free shipping
• Results in just 2-3 days from lab receipt of samples
• ISO 17025-accredited lab
• Only a simple cheek swab required – this can even be done on new-born babies
• Cost of the test includes your test kit and all lab fees
• Option to test up to five siblings  
• Zygosity certificate included – a unique keepsake for multiple birth parents/siblings
• We’ll NEVERshare your DNA results or data with any third-party organisations or DNA matching services

Have questions about the test? Call our friendly, knowledgeable Customer Services team on 01 402 9466 or email info@alphabiolabs.ie for more information.