I don't think that when Kirk saved Eleen, he was thinking "I need to ensure this baby is born so that there is a legitimate claimant to the throne and Maab's claim will be invalidated." I think he was thinking "I can't let them execute a pregnant woman because that's just wrong." He was imposing his personal and cultural morals onto a society with different morals -- and specifically he was forcing rescue on Eleen herself, who wanted no part of it. Of course personally I agree with his position, but the Prime Directive means respecting other cultures' right to their own laws and customs even when you find them abhorrent. Not to mention all the other starfaring civilizations out there that don't honor the Prime Directive and wouldn't hesitate to contact a pre-warp world.