One set of twins – two fathers: how common is superfecundation?

Heteropaternal superfecundation will be tackled in a new romcom – but it happens in real life, too. Two experts explain how

Constance Wu, of Crazy Rich Asians, is in talks to star in a new romcom. Apparently, her character is a bit ditzy – a classic romcom trope. But this ditziness will lead to what I am fairly sure will become a new romcom trope: heteropaternal superfecundation.

“It does sound a bit Mary Poppins,” says Michael Carroll, a reproductive scientist at Manchester Metropolitan University and the father of identical twin boys. He explains that heteropaternal superfecundation occurs when a woman gives birth to twins with different biological fathers. Once you can get past all the polysyllables, the romcom possibilities are endless: mistaken identity, twins who look comically different (see Arnold Schwarzenegger and Danny DeVito in Twins), competing suitors.

Although rare in humans, heteropaternal superfecundation is common in dogs, cats and cows, Carroll explains. “Females will have multiple matings with multiple males and this increases the chance of them producing multiple offspring.” Humans, on the other hand, “are not the best at breeding”. That is one way to look at it. So how does superfecundation with multiple fathers happen at all?

There are two ways through intercourse, according to Jason Kasraie, the chair of the Association of Clinical Embryologists. First, a woman can release two eggs at the same time. Since sperm can survive for a few days in the female reproductive tract, loitering in the corner of the womb and the fallopian tube, it would be possible to have sex with one father-to-be in advance of the egg being released, and another just after ovulation.

In the second scenario, the woman releases two eggs a few days apart but in the same reproductive cycle.

Either way, “It’s extremely uncommon,” Carroll says. “It all adds up to many rarities happening in the same cycle.” A sperm’s journey is arduous at the best of times. To have two successful candidates from two different men in a month when two eggs happen to be released … Well, what are the odds?

Kasraie has found two studies of the incidence of heteropaternal superfecundation, both from the early 1990s. One author claims that one in 400 pairs of fraternal twins (those arising from two eggs) fits the description. The second author puts the figure at one in 13,000 cases of paternity. These sound like best guesses. “They do,” Kasraie agrees. He points out that we cannot know the true number, and research has been scant. Most cases come to light only when paternity is questioned and a DNA test carried out. Apologies if this turns out to be a spoiler.
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