If you know nothing relevant, you're ambivalent between exchanging and not exchanging, which is what "fungible" means.
I see where you're coming from, but I'm not sure that's a useful use of the concept of "fungible". The reason why it's useful to think of dollar bills as fungible is because we can use them to think about more complicated exchanges (i.e. dollars -> cow -> cow + milk -> cow + dollars -> ...) and figure out a net result by comparing before and after quantities of the highly fungible little notes. Sure, it's possible that a given dollar might turn out to be counterfeit, but it only takes a little knowledge and a few moments of examination to become very confident about whether or not that's the case for a given note.
On the other hand, I'd be very reluctant to use babies as a unit of exchange even if I ignored the obvious moral problems: babies are much much harder to compare for equality than dollars.
A few years ago, I wrote a little dialogue I imagined between 2 materialists, one of whom was for and one against abortion, centering on the personal identity question. I recently cleaned it up and added a number of references for the biological claims.
You can read it at An Abortion Dialogue.
Early feedback from #lesswrong is that it's a 'nicely enjoyable read' and 'quite good'. I hope everyone likes it, even if it doesn't exactly break new philosophical ground.