&imwidth=800&imheight=600&format=webp&quality=medium "They thought they were identical sisters, DNA test revealed different fathers; UK records first case of rare biological phenomenon")
Michelle and Lavinia Osbourne have always known they were twins. They were born within minutes of each other, to the same mother, but it turns out they don't share the same father. The Osbourne twins didn't find out this shocking truth until they took a DNA test. They were conceived naturally, grew together in the same womb, and were born to the same mother within minutes of each other, but they are half-sisters. But how is that even possible? The answer is a rare phenomenon called heteropaternal superfecundation.
UK's first recorded case of twins with different dads
The 49-year-old Osbourne twins are officially the United Kingdom's first recorded case of heteropaternal superfecundation, a biological phenomenon so rare that fewer than 20 cases have ever been documented worldwide.
Speaking on a new series of BBC Radio 4's The Gift, the sisters told journalist Jenny Kleeman how they came to learn the truth about their origins. "We're miracles," they said. They came to know about their origins as recently as 2022.
What is heteropaternal superfecundation?
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Heteropaternal superfecundation occurs when a woman releases more than one egg during a single menstrual cycle, the same process behind fraternal twins, but the eggs are fertilised by sperm from two different men.
For this to happen, two acts of intercourse with different partners must occur within a roughly five-day window, since sperm can survive in the reproductive tract for up to five days while eggs remain viable for only 12 to 24 hours after release. The timing has to align almost perfectly, which is part of why it happens so rarely.
Most cases are never detected at all. The phenomenon tends to come to light only when twins look noticeably different from each other, different skin tones, for instance, or when a paternity dispute triggers DNA testing.
One in 400
A case in Brazil in 2022 became international news when a young woman discovered that only one of her twins matched the man she believed to be the father. Further testing revealed two different fathers. Cases like that one are typically stumbled upon by accident rather than by design. Estimates suggest heteropaternal superfecundation might occur in roughly one in every 400 fraternal twin births in the United States, though the vast majority go undetected and unrecorded.
