A New Jersey mum who recently applied for child support found out that the man she was bringing to court only fathered one of her twins.
The other baby — born at the same time as the first — had a different father.
It’s a phenomenon known to the medical community as heteropaternal superfecundation (hetero meaning different, pater meaning father, and fecund meaning fertile).
Here’s how it can happen: Two male partners have sex with the same female partner. One man’s sperm fertilize one of the woman’s eggs while the other man’s sperm fertilize another one of her eggs. This all has to happen in less than a week, doctors estimate, since sperm are only viable for about five days.
It sounds crazy, but it’s not as strange as it sounds, Columbia University assistant clinical professor of maternal and fetal medicine Russell Miller tells Business Insider.
Think about your standard case of fraternal (non-identical) twins, suggests Miller. In this scenario, two different eggs from the same female partner get fertilised by one male partner. Two of the woman’s eggs, two of the same man’s sperm. In heteropaternal superfecundation, the same thing happens, only instead of two sperm from the same man fertilizing two different eggs, two different sperm from two different male partners are fertilizing two eggs.
“In this case you have two different sperm, one for each egg,” says Miller.
How rare is this?
Doctors assume that heteropaternal superfecundation is infrequent, but it’s hard to tell for sure, Miller says.
Most known cases of the phenomenon have only been brought to light for legal reasons, or when someone orders a paternity test. A 1992 study, for example, found that amongst all the cases involving twins where one partner was questioning the paternity of another, superfecundation had occurred about 2.4% of the time.
But that statistic can’t be applied generally to the rest of the population because it’s already looking only at people who’ve taken their case to court or sought legal assistance.
Another reason doctors presume this phenomenon is so uncommon is because they rarely do paternity tests in the first place, says Miller.
“The truth is nobody knows,” Miller says. “I mean we presume this is rare, but we don’t know at all.”