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it doesn't look like the neat-o third arm actually moves, though. At least it never did in the CNN snippet they showed of beautiful NextGen Jie-Jie Baby in his crib.
I say keep it even if it doesn't move, and maybe he can one day have a child of his own with one that does.
Not to be gross or depressing, but there's most likely a sizeable unseparated twin inside this baby, whose left arm is one of those we can see on the outside. Depending on how integrated the twins still are (blood vessels, nerves, etc) the shadow twin will either survive or not- if it does not, this little fellow doesn't stand a chance.
If you look at him picturing one baby inside another, you can tell which arm is his and which is the twin's. That doesn't determine which arm the docs should try to keep... it all depends on who grew the most nerves and blood vessels and so on into their arm, and whether that arm can be connected to the external baby's brain and circulatory system.
Then, like I said, the internal twin has to stay alive if there's any hope for the external one to survive.