In contrast, babies are basically fungible.
This doesn't follow. Just because you don't have data about what counterfactually the baby would have turned into doesn't mean that the babies are fungible. We don't in general keep the genetic code of aborted babies or calculate how that would interact with their environment. Just because we can't easily predict what the distinctions would be doesn't mean that the babies are all identical.
When you're considering a decision of exchanging two babies, you're making it based on what you know to anticipate. If you know nothing relevant, you're ambivalent between exchanging and not exchanging, which is what "fungible" means.
(The dollar bills are also not identical, and where one bill can buy you a snack, another won't work by being suspected counterfeit in a manner you didn't expect. Such considerations don't make cash non-fungible.)
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.