Sex-ratio in Siamese twins.

Paul Kemp pkemp at
Fri Mar 5 15:51:34 EST 1993

I heard an interesting fact the other day, and don't have a good explaination
for it.  80% of Siamese twins are female.  Does anyone know why this is true?

I am assuming that Siamese twins are identical twins.
I believe the sex-ratio in identical twins is pretty close to 50:50

Possible causes:

1. Male Siamese twins die before they are born.
2. Females are pre-disposed to incomplete fission.
or 3. (the interesting one) Some Siamese twins are XY, but due to incomplete
      development they never obtain male characteristics.

Anyone have any ideas?
pkemp at

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