Consider the case where a mother is Shected and a viable child (Ben Pekuah) is found inside. The Rambam holds that the flesh of the Ben Pekuah is permitted (from the Torah) without Shchita. However, the Chelev and Gid Hanashe, prohibited in other animals, will also be prohibited in the Ben Pekuah. The Ramban disagrees and states that both the Clelev and Gid Hanashe are permissible in a Ben Pekuah.
Tosfos and Rashba permit only the Chelev but not the Gid. The Kreisi Uplaisi explains the reason why the Gid Hanashe is different from Chelev. Chelev is permitted because the Torah says whatever is in the animal may be eaten, from which he learns the Ben Pekuah itself is permitted along with it's Chelev. However, in this case he holds the blood is forbidden. One of the reasons for this comes from the Rishonim, who permit the flesh to be eaten, but do not include drinking, which they hold is prohibited. Therefore blood which is drunk would be forbidden.
We can apply the same reasoning to Gid Hanashe. Since we Paskin that the Gid is tasteless and therefore not edible (and even though it's essentially non-edible, however the Torah stills prohibits doing the action of eating it), we cannot include the Gid Hanashe in the Heter of eating what's in the mother.
