The gemara implies that a "ben yom echad" is considered a life so that if one would kill him, the killer would deserve capital punishment. Tosafos explains that this would not apply to a fetus. Prior to birth we wouldn't render a fetus a full fledged life. When Tosafos discusses the issue of killin a fetus - abortion, they use a language of "mutar" - permitted. The simple reading of Tosafos would indicate that not only would we exempt one who aborts a fetus from capital punishment but it would actually be permitted to abort a fetus. Tosafos asks that it is clear from the gemara in Erchin 7b that one is permitted to violate shabbos to save a fetus inside the mothers womb. How can we permit the violation of shabbos for a fetus that one is "allowed" to kill? Tosafos answers that although one must even violate shabbos to save a fetus due to the importance of pikuach nefesh (saving a life), nevertheless it is permitted to kill it. Tosafos cites a proof from one who is a go'ses - dying, where we find that one who murders him is "patur" - will not receive capital punishment, but it is still necessary to violate shabbos to save him.
If we are to take Tosafos at face value, that "mutar" actually means permitted, the logic of Tosafos is impossible to understand. How can it be permitted to kill the fetus, yet would warrant violating shabbos to save it for pikuach nefesh?! If we don't regard it as a life and permit the killing of it, we certainly can't justify the violation of shabbos to save a "life"! Furthermore, the proof that Tosafos cites from go'ses wouldn't prove this chiddush, it would only prove that you could violate shabbos for an individual that you wouldn't receive capital punishment for killing, but you couldn't prove that you could violate shabbos for one who it is "permitted" to kill. For this reason, Rav Moshe writes in a teshuva that the language of Tosafos is misleading. Tosafos never meant that it is actually "mutar" - permitted, to kill a fetus, rather Tosafos is just saying that one wouldn't receive capital punishment for doing so. Rav Moshe understands that it would be a Torah prohibition to abort a fetus and would even fall under the heading murder, but it would be the type of murder for which there is no capital punishment. According to this approach it is understandable how one can violate shabbos to save a fetus. Tosafos thought in their question that the right to violate shabbos should be dependent on their literally being capital punishment for aborting. Tosafos concludes that even if there is no capital puishment, since it is still considered a life by the torah so that one who aborts would be considered a murderer, it is permitted to violate the shabbos to save the fetus. This approach is supported by Tosafos in Chulin 33a that distinguishes between goyim and jews - a goy is killed for abortion just like actual murder, whereas a jew is not killed, which implies that it is still a prohibition. This approach is against the chavos yair cited by maharatz chiyus who says that prior to labor it is literally permitted. Rav Moshe considers the approach of the chavos yair to be a terrible error.