The Pro-life View in Conjunction With the Thesis That Replacement Is Bad Is Implausible
A counterexample
Suppose you think that life, by which I mean the time in which an entity starts to matter, begins at conception. You also think that for living humans, they deserve rights — these include the right not to be replaced with others that will have higher utility. It follows that it would be bad to replace embryos — at conception — with other embryos that will have higher welfare.
This, however, has a rather unintuitive result. Suppose that embryos are replaced 100^1000^1000 times, leading to each sentient being that exists gaining more welfare per second than all humans ever have so far. On this account that would be a terrible thing — each replacement only marginally increases welfare and causes an objectionable displacement. But this is very implausible.
I object to this the same way I object to objections of the repugnant conclusion: it's hard to have intuitions about numbers that large. Is it really possible to image someone "gaining more welfare per second than all humans ever have so far"?
But this is very plausible. No one would say that the marginal displacement each time is worth it. I wouldn’t kill 10^10^1000 humans to create a utility monster.
This leaves aside the bit about fetus rights.