The gemara says in temurah 30b that when a person is rovei'ah a pregnant cow, the cow and calf are both chayev misah because היא וולדה נרבעו. Tosafos explains that this does not prove that the child is viewed as a limb of the mother since the chiyuv misah on the calf is due to the calf also receiving "hana'ah" from the act of aveira. However, our sugya which says that the damage caused by pregnant cow can be collected in full either from the cow or from the calf, indicates that the child is viewed as a limb of the mother, otherwise each should only be responsible for their share and not for the entire damage.
R' Akiva Eiger (kesavim 172) has a famous question that if we assume that היא וולדה נרבעו because the calf is a limb of the mother, why would it be permitted for one to have relations with his pregnant wife - shouldn't it be a violation of having relations with one's daughter? R' Akivah Eiger considers the assumptions in the question to be valid, and therefore answers based on a technicality that since the fetus who is his daughter is less than 3 years old, it is not viewed as an act of relations. Although one would be forbidden to have relations with any issur ervah who is less than 3, that is not a violation of an issur ervah, rather a violation of wasting his seed, which does not apply when he is having relations with his wife.
Focusing on the question of R' Akiva Eiger, which he prefaces by saying that if עובר ירך אמו then it should be forbidden to have relations with one's wife who is pregnant - why does he preface the question with the concept of עובר ירך אמו since tosafos explains that the din of היא וולדה נרבעו applies even if עובר לאו ירך אמו based on the concept that the fetus also gets hana'ah?
Also, the assumption of R' Akiva Eiger that when one is roveiah the cow, it is as if he is also rovei'ah the unborn fetus seems strange. In the case of the cow and calf, since the calf is considered a limb of the mother, it becomes assur together with the mother even though the bi'ah was to the mother and not the calf. But in the case of a man having relations with his pregnant wife, it is not as if he had relations with the daughter, so there is no reason to assur the act. It may be true that if the woman would assume a status of issur, that may apply to the child just as the status of "nir'vah" applies to the child. Based on this, the question of R' Akiva Eiger would actually be stronger if we would say עובר לאו ירך אמו and therefore be forced to say that the issur on the calf is because it also received hana'ah , implying that it is as if he actually had relations with the calf also?