The Rambam apparently contradicts himself in terms of עובר לאו ירך אמו [a sheep's fetus is not part of his mother.]
For one one hand he writes When a person who separates a pregnant sin offering (sheep, goats, or cows), the mother or the infant can be brought as a sin offering. Quite openly saying the law that עובר לאו ירך אמו a fetus is not part of his mother. Yet in laws of Temura replacement he writes when one separates a pregnant female for a Passover sacrifice when she gives birth they both go to pasture until they get a blemish naturally, and then as sold and with that money a Passover offering is brought. Or if she gives birth after Passover, then both are sold for a peace offerings. The reason the for this last law is the exact opposite of the reason for the first law עובר ירך אמו a fetus is part of his mother.
[This last law is subject to a debate. R. Elazar says when one separates a pregnant sheep for a Passover, if she gives birth before Passover, the infant is brought as a Passover sacrifice and the Gemara says the reason for R Elazar is עובר לאו ירך אמו [a sheep's fetus is not part of his mother. So we see the sages hold עובר ירך אמו a fetus is part of his mother.
[Contradictions of this kind are very common in the Rambam and that gives plenty of folder for debate as to why. (Mainly held the best idea is to go with the simple approach of the Gemara like the Rosh. But others have tried to find explanations for these contradictions to the Gemara that you see in the Rambam all the time. ) Mainly the idea that Rav Shach brings is the mother and fetus are possible sin offerings. While in the case of the female separated for a passover sacrifice, the mother is obviously not going to be a passover --the passover must be a male. Okay--that makes some sense. The problem is if עובר לאו ירך אמו [a sheep's fetus is not part of his mother then why should this matter? Even if the holiness that descends on the mother is a pushed off holiness (only applicable in terms of money) still fetus should be a regular passover sacrifice just as when sets aside a male sheep for holiness of money, still automatically it becomes set for a regular sacrifice.
I am hoping to go "vitter" further to the next sugia, but ust for a last note, it seems Rav Shach does agree with this idea that since the holiness that devolves on the mother is pushed off, that make the born sheep also not fit for a sacrifice. I noted tat he brings one of the Baali HaTosfot as a proof to this.
