Temurah 30b - Having relations with one's wife while pregnant

Rava says that if one were to commit bestiality with an pregnant animal, the fetus will also assume a status of "nirva" and be forbidden to be brought as a korban because היא וולדה נרבעו. The gemara explains that this issue is dependent on the notion that the child is a "limb" of the mother - עובר ירך אמו הוא, therefore anything done to the mother is as if it were done to it's child. Based on the same rationale, if a pregnant animal kills, the fetus is also considered a "no'geiach" and killed. Since the fetus is part of the mother it is as if it participated in the actions of the mother. 

R. Akiva Eiger (Kesavim 172) asks a very strange question. Based on the rationale of the gemara that היא וולדה נרבעו, it should be forbidden for one to have relations with their pregnant wife since it is tantamount to having relations with their own child? R. Akiva Eiger offers a technical answer. Since chazal don't consider it to be an act of relations when a girl is less than 3, it wouldn't be a torah violation. Although it would normally be a Rabbinic violation to have relations even with a girl less than 3, the violation is based on wasting seed which obviously doesn't apply when he is having relations with his pregnant wife. In short, R. Akiva Eiger considers having relations with one's wife while pregnant to be having relations with the fetus itself.

The Yachin Bo'az says that R. Akiva Eiger's entire approach doesn't make sense. If we truly consider every act done to the mother as if it were done to the child, there should be a violation to shecht the mother since it is as if he were shechting the mother and child on the same day. The Tiferes Yisroel (Boaz) explains that the answer lies in the gemara's assumption that עובר ירך אמו. Since the fetus is considered part of the mother it is not considered to be having relations with the fetus, just with the mother. The concept of היא וולדה נרבעו doesn't mean that it is as if he committed the aveira with both the mother and the child, but rather since the child was part of the mother at the time of the aveira, it assumes whatever status the mother has. The concept of עובר ירך אמו הוא doesn't make it as if he were having relations with the fetus, rather it makes that the relations he had with the mother causing it to assume a status of a נרבע would also apply to the fetus. In a situation where there is no status placed on the mother, there is no effect at all on the child. According to this approach the phrase היא וולדה נרבעו isn't to be taken literally, it is merely an expression meant to convey that the status of the mother will apply to the child as well.