Abstractly, at what stage of development is the suffering of 100 fetuses commensurate with the suffering of a newborn? 1 month of gestation? 4? 9?

Concretely, how many people that you know have faced a situation where that calculation is relevant?

More concretely, if you're pregnant, you'll have to decide not only whether the phenomenological point of view of your unborn child should be taken into account in your decisionmaking, but you'll have to decide in what way and to what degree it should be taken into account.

I don't know how much you'll have to decide on how you decide. You'll decide what to do based on your valuations - I don't think the valuations themselves involve a lot of deciding. I don't decide that ice cream is yummy; I taste it and it is.

And yes, I think it's a good policy to review your decisions and actions to see if you're endorsing the choices you're making. But that's not primarily an issue of suspect ontologies, but of just paying attention to your choices.