This is one of those issues where the Rambam seems to contradict the Gemara directly. Yet there are ways of answering for him.
The issue is this. The Rambam decided the law that if the mother sheep was pregnant or not, the sheep that was born is sold.[The money is then used to bring a peace offering]. This is not like R. Elazar who said in the case she was pregnant (at the time of consecration), the born sheep can be brought as a Passover offering. Ravina [in Temura page 19 side a] says the reason for R Elazar is the sheep that has not been born yet is thought to be separate from the mother. So it certainly looks that the reason the sages said the born sheep can not be brought is that they hold the sheep in the womb of the mother is considered as part of the mother.
Yet the Rambam decided like the sages in Temura pg 19, but also that the unborn sheep is separate from the mother.
This looks to be a contradiction. Rav Isaac Zev [son of Rav Chaim of Brisk] says to answer the Rambam we can say the sages agree that the unborn sheep is not part of the mother, [in spite of the clear implication of the Gemara]. But that just like when the mother was not pregnant yet and then became as such then the unborn is dragged along with her category--to be sold and the money used to buy a peace offering. So they say the same thing even when she was already pregnant.
Rav Shach asks the question that this clearly not like the opinion that the unborn is not part of the mother. He attempts to find a different answer for the Rambam. --that the fact that the mother sheep could only be sanctified as far as monetary values goes, that is what causes the unborn sheep also to be dragged along with that.
Here the Rambam seems to be at odds with the Gemara. It would be simpler to say that the sages hold the unborn sheep is part of the mother. But the Rambam is interested in the law, and so here he is wondering what R. Yohanan would answer for the sages. And he has a reason to say the law is like R. Yochanan, that the unborn sheep is not a part of his mother.
We know how Ravina explains the argument between the sages and R. Elazar. But how would R Yochanan explain it? Clearly he would not say that the sages disagree with him. So we come to this idea of Rav Shach that the act declaring the mother sheep to be Passover offering which can only mean monetary value must transfer to the unborn sheep also.
