Follow me through, here.
I would imagine that it’s - however rare - technically feasible that a woman could have TWO eggs be fertilized on the same day by two different men. I don’t know of any cases where this has actually happened, but it seems that, on some occasion, the fallopian tubes could produce two eggs, that this woman could have sex with two different men in one day, and that those two eggs could be fertilized by the two different partners on that day.
It’s happened, it’s called Superfecundation. It’s unlikely. Fraternal twins are uncommon to begin with (1 in 90 in the US), so you have to figure the odds of one woman producing two ova at the same time, then having sex with two fertile men within the right time frame (I won’t hazard a guess here). Then the odds that each guy will fertilize each egg (probably less than 50-50).
I’ve heard it happens a lot with cats.
Cecil on superfecundation.
it was estimated that about 1/400 twins may be from different fathers
from http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=7871943&dopt=Abstract
Does that mean 1 in 90 sets of twins? I’m the father of fraternal twins (one boy one girl), and I’m surprised to see it’s that unusual.
I have a pair of friends who look exactly the same in every way, but who claim that they are fraternal, not identical, twins. Is that possible?
Sure. Apparently, the Olsen twins are actually fraternal. Go figure.
No, I think it’s meant that 1 in 90 humans born in the US is a fraternal twin.
Or that one in 90 pregnancies is fraternal twins, which is significantly different from 1 in 90 persons.