As soon as they were born, family members noticed something strange about them.
Twins are rare, of course, but there was something very unique about these girls, born in Vietnam in 2013.
One of the girls had thin, straight hair, while the other had significantly thicker, wavier hair. Their facial features were markedly different. As they grew older, the differences between the children became more pronounced, and family members began to demand answers.
Had the hospital made an unthinkable error? Was one of the twins switched at birth?
The mother, identified only as T.M., underwent genetic testing; that’s when doctors found something extremely surprising. The little girls were, indeed, twins—but they had different fathers.
Now, these two girls are attracting international attention from the medical community.
They’re called bi-fraternal twins, conceived as a result of a process called superfecundation.
They have separate fathers, but, of course, the same mother. Like other types of twins, they were born simultaneously, and doctors were unaware of their rare circumstances at birth.
Twins are, of course, always something of a rarity, as only one in every 90 births results in multiple offspring (discounting births aided by in vitro fertilization). Bi-fraternal twins require a very specific set of circumstances, and as such, are much more uncommon; only a few cases have been positively identified.
Here’s what happens: Instead of releasing a single egg, the mother’s body releases two eggs, either at the same time or at different points in her cycle. Sperm can live in a woman’s body for up to five days, so a different sperm fertilizes each egg.
“There are less than 10 known cases of twins with different fathers in the world,” Le Dinh Luong, President of the Genetic Association of Vietnam, told CNN. “There might be other cases but the parents and/or the twins were not aware of it or didn’t want to announce it.”
One of the other cases occurred in New Jersey, where a child support case revealed that a set of twins had two separate fathers.
The county had brought the case on the mother’s behalf, arguing that the father of one of the twins should make child support payments for both children.
However, genetic testing proved that the man was only biologically related to one child. The judge in the case ruled that the father involved in the case would only be required to pay child support for his biological daughter—not the other twin.
“Today, it is believed that the incident of bi-fraternal twins is increasing at a higher rate compared to 50 years ago, mainly due to assistive reproductive technologies, ovulation induction, promiscuity and other factors,” wrote Passaic County Superior Court judge Sohail Mohammed when delivering the decision.
However, Mohammed’s analysis of the situation may be somewhat misguided. Most bi-fraternal twins are discovered through genetic testing in paternity cases like the one in New Jersey, which may explain why more cases have been identified over the last several decades.