PART 1 of 2
When Mia Washington and her boyfriend James Harrison ran a paternity test on her eleven-month-old twins, they got a result that made news at home and abroad. Harrison was the father – of only one twin. The other dad? His identity has not been released. Washington told Fox News: “Of all the people in America and of all the people in the world, it had to happen to me. I’m very shocked.”
How rare is this? To no one’s surprise, there isn’t a lot of research on this subject (Can you imagine doing the informed consents for that study?). However, estimates suggest that as many as 12% of all fraternal twins are conceived in two separate acts of coitus, a phenomenon common enough to have its own six-syllable name: superfecundation. Parenthetically, this raises an interesting question: how many genetic counselors routinely consider the possibility that twins may differ by days or even a week in their gestational age?
And if mommy is spreading the love, there is no guarantee that the resulting children won’t have different fathers (“heteropaternal superfecundation”). In fact, a review of one database of paternity test results revealed bi-paternity in three cases, or 2.4% of all fraternal twins tested. You have to assume a major ascertainment bias in a population doing paternity testing, so it is hard to know how to generalize those numbers. One study suggests that of all naturally conceived fraternal twins born to “married, white women in America,” one in 400 sets are bi-paternal. The author adds that the number may be higher in certain populations, “like prostitutes”. Going out on a limb there, buddy.
Bi-paternity may be news but it is a pretty safe bet it isn’t new. As shocking as it is to the rest of the world, to genetic counselors it has a familiar ring to it – another cautionary tale about how treacherous it is to make assumptions about paternity. But there is a more ancient angle to this story as well; more on that in my next post.