My wording in the grandparent was misleading. This:
The 4 month old will produce 80 years of utility if saved, for 18 years of investment: if this is a bad bargain, reproduction always is.
Was meant to refer only to having a baby in current conditions regardless of whether or not it would be sacrificed to save an adult. I meant that if 80 years of production minus 18 years of investment was a net negative or a net positive, that wouldn't change whether you saved the adult or the baby.
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.