RSZA is quoted in the Nishmat Avraham Choshen Mishpat 425 Note Aleph:1. He says as follows (my own translation):
"In my humble opinion it seems to me that just as it is prohibited to steal the property of a fetus, just the same it is prohibited to steal from him his life. It is the opinion of Tosfos in Masseches Bava Basra 142a D'H "Ben" that a fetus inherits while he is still in-utero, even if he will in the end be a "nefel" (a fetus which does not survive - usually due to prematurity)."This is a very interesting suggestion, and at first it seems quite compelling. However, one must wonder why no other posek until HaRav SZA ever suggested this as a potential reason to prohibit abortions, despite the incredible amount that has been written on the subject by the Rishonim and the Acharonim.
One possible reason why no other poskim have suggested this, is because it would get us involved in a basic machloket among the poskim regarding the shita that HaRav SZA brought in the name of Tosfos. It is a fact that other Rishonim disagree with Tosfos, See the Rif in Bava Basra 142a who paskens that a fetus does not inherit, and the Shach in Choshen Mishpat 210:1 paskens like the Rif, not like Tosfos. An entire discussion of this topic is beyond our scope, but certainly it may well be that since many poskim hold Halacha lemaaseh not like tosfos, therefore they held that one cannot steal from a fetus, rendering this entire argument a non-starter.
It also seems that this issue revolves around a fundamental question. Let us assume like the shita of Tosfos, that it is possible to steal from a fetus. One still needs to ask as follows: Does a person own his/her life in the same way that they own property? This is to say, if a person is a victim of murder, can we say that the murderer is also guilty of stealing?
Of course, since the perpetrator is liable for the death penalty, beis din cannot collect money from him, due to the familiar rule of Kam Ley Mi'drabbah miney (KLMDRM). That is, since beis din can only mete out one punishment, and the rule stipulates that he can only get the harsher punishment, which in this case is death, not payment. But theoretically, is there also a monetary claim against the perpetrator?
Interestingly, there is a source for this question among the Rishonim. The Hagahot Ashri, in Bava Kama Perek 4 writes as follows (my own translation):
If Reuven killed Shimon, even though he is liable for the death penalty, he still pays the estate (of Shimon) his value in order to exempt himself from the obligations of Heaven ("La'Tzet Yedei Shamayim"). Or one may say, that if the inheritors of Shimon were to confiscate the money from Reuven, we would not demand that they repay it to Reuven ( "ee tafsi yorshim miney, lo mafkinan minayhu").So according the Hagahot Ashri, although the beis din cannot collect it from Reuven, there is a monetary claim against him. This proves that a person does "own" his life in the sense that one who takes it from him, has also taken away a possession of his. This clearly supports the contention of HaRav SZA.
However, curiously, this halacha of the Hagahot ashri is not codified in any of the rishonim or acharonim le'Halacha. It seems that it was not accepted by any of the major poskim, and the question of course is why.
There could be two possible reasons why the other poskim do not mention it.
1) They really agree with the Hagahot Ashri in principle, just they did not feel the need to codify it (Either because they did not see any need to discuss payment which is only "Latzet Yedei Shamayim" or because they disagree regarding the Hagahot Ashri's idea that if it was confiscated by Shimon's family that Beis Din would not force Shimon's family to pay it back)
2) They really disagree with the Hagahot Ashri's idea that a person "owns" his/her life in the same way that they own property. maybe they feel that this entire concept is incorrect, and a person does not own his/her life, and though one who murders is liable for the death penalty, there is no monetary obligation incurred. If this is the case, there would be no support for RSZA's opinion, in fact, it would be impossible to suggest that gezeilah is a possible reason for the prohibition of abortion.
Before we leave this topic, we must point out another interesting question that arises from the suggestion of HaRav SZA. that is, if it is possible to steal from a fetus, can the parents give permission to take what belongs to the fetus? In other words, we do know that what a minor owns is owned by the father, so does this apply to his/her life as well? In practical terms, if this is the reason to prohibit abortions, can the parents give permission to take away the fetus' most precious possession, his/her life? I only ask this question, but I don't have a practical answer. Once we treat the fetus as a being that owns things, we should apply the rules of ownership consistently and give the parents full rights over the "property" of the fetus. However, the thought that the parents own the life of the fetus to the extent that they can give such permission, is more than a little chilling. I have no answer, just food for thought.
Instead of reviewing all of the reasons regarding the prohibition of abortions, I will refer you to look at the end of this post for the first six, and I will add numbers 7 and 8 here:
7. According to the Ohr Sameakh's understanding of the Rosh, one who performs an abortion without the permission of the parents is transgressing the prohibition of gezeilah, stealing.
8. According to HaRav SZA ZTL one who aborts a fetus is guilty of gezeilah from the fetus him/herself. Whether or not the parents would be allowed to give permission remains an open question.
Thank you for hanging in there as we go through this lengthy and detailed analysis. Next we will discuss the prohibition of Hotza'at Zerah Levatalah or "wasting seed". This is usually associated with masturbation, but we will find that some major poskim have applied some of those rules to abortion as well. We will then move on to various miscellaneous reasons, which i will bundle into one category which you will understand when we get there, and then we will move on to the most stringent opinions, those that hold that the prohibition of retzichah (murder) applies to a fetus. That will complete our analysis of the opinions, and then we can move on to the next step. That's when it'll get even more fascinating, so please stay with me for the fun part!