the argument against killing fetuses is theological (unbaptized, go to hell) but it can be reconstructed as a secular argument (no agency, unable to be considered a moral actor) the argument against doing things to embryos is that they have no agency.
presumptively positive interventions might not be, so hesitancy to perform them seems reasonable. there's also another basically theological position related to purity/bioconservatism being a relevant marker that would harm someone to deny them pre-birth.