Twin twins
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Drugs aren’t ordinarily involved in the formation of identical twins. They can be involved for fraternal ones, though, because fertility drugs tend to make it more likely that the mother will release two (or more) eggs in a single ovulation cycle. If each were fertilized, the resulting twins (or triplets, or other multiple births) would be fraternal. In contrast, identical twins come from a single fertilized egg that splits in two very soon after fertilization.
In this case the mother didn’t take fertility drugs. But she must have released two eggs anyway, which were then fertilized by one sperm each. Then, instead of developing into a single set of fraternal twins as would ordinarily happen, each fertilized egg split to form the two sets of identical twins. No wonder it’s rare!
Cain and Abel are thought to be the earliest example of superfetation.
I had to google “superfetation” and found out -yet again- that things are stranger than I knew.
I think in my day we would call them quadruplets. We know now that these were two sets of identicals but I wonder how many there have been in the past before anyone thought to call them two sets of twins instead of quadruplets?
Ruth H:
It’s possible, but IMHO highly unlikely. Identical twins look almost exactly alike, whereas fraternal twins (even of the same sex) look no more alike than ordinary siblings. It would be the highly unusual set of fraternal twins that would resemble each other strongly enough to be seen as identical.
So, if a woman had identical quadruplets, that would certainly be noticeable and differentiated from fraternal quadruplets. But two sets of identical twins at the same time—that would tend to be noticeable and remarkable, too, and to be differentiated from either identical quadruplets (very rare incidence of 1 in 15 million) or some other combination of babies that would give four at the same time.
It’s hard to figure out how common spontaneous conception (as opposed to assisted) of quadruplets used to be, but it certainly was very low as well. Here are some current facts:
The site also says that 418 sets of quadruplets were born in 2005. If 90% of those were fertility-assisted, that means that only about 30 of them had been spontaneously conceived.