Rav Shach asks this question on the Rambam.[laws of temura, chapter 4 law 7] If one sanctifies a female sheep to be a Passover sacrifice (and we know a female cannot be a pesah) and it gives birth to a male sheep, both go to pasture. This like the first tana in Temura page 19a. But the Gemara says that this law of the tana is applicable even if the female was pregnant at the time she was sanctified, and the gemara says the reason is because the baby in the womb is part of the mother. (The offspring does not become sanctified separately from the mother.) However, the Rambam decided the law like this tana, and yet also holds that the baby in the womb is not part the mother, so the Rambam should have decided the law is the male offspring can be sacrificed as a Passover like R. Elazar. To answer this question I would like to suggest that in Tmura page 10 there is an argument between R. Yochanan and Bar Pada. There, Bar Pada said holiness does not descend on a baby in the womb. I think the Rambam decided the law to be like Bar Pada. [Therefore, even though the baby is not part of the mother still it cannot be sacrificed as a Passover offering]. {But one can still ask on this because the Rambam says the baby must not be sacrificed as a Passover, while Bar Pada might mean that holiness does not automatically descend on the baby, but that after it is born, it can be sacrificed for whatever one intends it to be.} However, he interprets Bar Padaa to mean that a baby in the womb can only receive holiness of money not holiness of body. and we can see many examples of this in the Rambam for example if one sanctifies a baby in the womb to be a sin offering. There also the holiness that descends on the baby in that of moeny.ie it goes to pasture until it receives blemish and is sold and the money is used to buy a sin offering. The question on this would come from the law that the Rambam writes that a baby in the womb of an animal with a blemish is considered whole and perfect and can be sacrificed I think the way people understand the rambam is that if one sanctifies a female animal to be a peace offering and then it receives a blemish then its offspring can still be sacrificed to be a peace offering. but I think the Rambam meant that the offspring can be sacrificed as a peace offering but not that it is sacrificed as such. rather one needs to sanctify the offspring after it is born and then it can be sacrificed a s peace offering. Rav Shach brings the answer of Rav Isaac Zev, but asks on it. And he does not find a better answer. So here I suggest a possible approach to answer this question. ---------------------------------------------------------רב שך asks this question on the רמב’’ם. If one sanctifies a female sheep to be a קרבן פסח (and we know a female cannot be a פסח) and it gives birth to a male sheep, both go to pasture. This like the first תנא in תמורה page י''ט ע''א. But the גמרא says that this law of the תנא is applicable even if the female was pregnant at the time she was sanctified, and the גמרא says the reason is because the וולד in the womb is part of the mother. (The וולד does not become sanctified separately from the mother.) However, the רמב’’ם decided the law like this תנא, and yet also holds that the וולד in the womb is not part the mother, so the רמב’’ם should have decided the law is the male וולד can be sacrificed as a פסח like ר' אלעזר. To answer this question, I would like to suggest that in תמורה page י' there is an argument between ר' יוחנן and בר פדא. There, בר פדא said holiness does not descend on a וולד in the womb. I think the רמב’’ם decided the law to be like בר פדא. [Therefore, even though the וולד is not part of the mother still it cannot be sacrificed as a קרבן פסח. {But one can still ask on this because the רמב’’ם says the וולד must not be sacrificed as a פסח, while בר פדא might mean that holiness does not automatically descend on the וולד ,but that after it is born ,it can be sacrificed for whatever one intends it to be .} However, he interprets בר פדאa to mean that a וולד in the womb can only receive holiness of דמים not holiness of גוף. and we can see many examples of this in the רמב’’ם ,for example if one sanctifies a וולד in the womb to be a sin offering. There also the holiness that descends on the וולד in that of money. i.e. it goes to pasture until it receives blemish, and is sold, and the money is used to buy a sin offering. The question on this would come from the law that the רמב’’ם writes that a וולד in the womb of an animal with a blemish is considered whole and perfect, and can be sacrificed. I think the way people understand the רמב’’ם is that if one sanctifies a female animal to be a peace offering, and then it receives a blemish, then its וולד can still be sacrificed to be a peace offering. But I think the רמב’’ם meant that the וולד can be sacrificed as a peace offering, but not that it is sacrificed as such. Rather one needs to sanctify the וולד after it is born, and then it can be sacrificed as peace offering.