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Old 11-20-2004, 11:37 PM
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Are opposite sex conjoined twins possible?

Are opposite sex conjoined twins possible? Could 2 fraternal twin embryos fuse in the womb, but instead of creating a single individual (a Chimera Twin) result in conjoined twins?
Old 11-21-2004, 12:59 AM
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The only conjoined twins I've ever heard of are not fused embryos, but rather derived from embryos that have not split completely. Simply, they're identical twins, originating from the same embryo, but unlike most identical twins, the halves of the original embryo don't completely separate.
Old 11-21-2004, 04:25 AM
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IANAOBGYN, but I'd say no. As I understand it, fraternal twins develop in separate amniotic sacs, which would prevent them from "fusing" once they had developed to the point where they begin to appear human. The only time something like that could happen is very early in development, when the embryos involved were only composed of a few cells. The scrambling that occurs when that happens usually results in a spontaneous abortion; chimeras only survive when their genes combine in such a way that they code for a normal human. I think that a zygote that was coded to have two heads, three arms, or etc wouldn't make it.

It is possible for a chimera twin to be both male and female, though they usually present as one sex.
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Old 11-21-2004, 06:50 AM
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Gender itself is not just genetics, but hormonal during development from what I understand, so you can have a XX male and XY female. So it should be possiable to have a male and female identical twin - though they might be taken as fraternal, I'm not sure if cojoined identical twins could have opposite gender since the blood is shared and the . It may be possiable if the blood sharing between the 2 are minimal.
Old 11-21-2004, 11:04 AM
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Are opposite sex conjoined twins possible?

Nope. Fraternal twins are separated by amniotic sacs and have separate placentas. Even if they grow atop one another, amnion and chorion are separate. The placentas may overlie each other, but circulation is kept separate. It is conceivable there could be some passage of blood cells or even stem cells by intimately overlapping placentas, but this would not result in fusion, just foreign tissue which the recipient will probably eventually minimize or reject completely as development continues, either pre or post birth.

If an XX set of conjoined twins is exposed to testosterone via the mother during the pregnancy, they will both become masculinized.

QtM, MD
Old 11-21-2004, 11:16 AM
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Though I should modify my reply a bit. You ask Are opposite sex conjoined twins possible? In nature, for humans, I don't believe they are. Thanks to modern biotech, it may be possible to construct such a situation.
Old 11-21-2004, 11:30 AM
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I seem to remember someone starting a thread on this subject before, based on Sherri Tepper's novel Sideshow. Two of the heroes were a set of opposite sex conjoined twins. However, they had actually been born as intersexed conjoined twins. At birth the nurses and mother decided that the ambiguous genitals of one of the babies looked more male than female, while the other looked more female than male. So the characters were raised as a boy and a girl respectively. They had the appropriate gender reassignment and cosmetic procedures later.

I think that would be the closest it would be possible to come to opposite sex conjoined twins, although AFAIK this has never happened in real life and would probably be a bad idea from both a medical and ethical perspective.
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