Arakhin 7B

מחתך בבשר הוא! אמר רבה: לא נצרכה להביא סכין דרך רשות הרבים.

It is merely cutting flesh, and there is no reason why it should be prohibited. Rabba said: No, the halakha concerning cutting open her abdomen is necessary only to teach that it is permitted to bring a knife by way of the public thoroughfare for that purpose, despite the fact that this constitutes a prohibited labor by Torah law.

ומאי קמשמע לן, דמספיקא מחללינן שבתא? תנינא: מי שנפלה עליו מפולת, ספק הוא שם, ספק אינו שם, ספק חי, ספק מת, ספק נכרי, ספק ישראל ⁠— מפקחין עליו את הגל!

The Talmud asks: And what does this teach us? Does it teach that even in a case of uncertainty we desecrate Shabbat for the chance of saving a life? But we already learned this in a Mishnah (Yoma 83a): With regard to one upon whom a rockslide fell, and there is uncertainty as to whether he is there under the debris or whether he is not there, and there is also uncertainty as to whether he is still alive or whether he is dead, and finally there is uncertainty as to whether the person under the debris is a non-Jew or a Jew, one clears the pile from atop him on Shabbat.

מהו דתימא: התם הוא דהוה ליה חזקה דחיותא, אבל הכא דלא הוה ליה חזקה דחיותא מעיקרא ⁠— אימא לא, קא משמע לן.

The Talmud answers: It is necessary to teach that one may bring a knife in the case of a woman, lest you say that it is specifically there that one may desecrate Shabbat, as the person who was buried under the rockslide had a presumptive status of being alive and therefore he is assumed to still be alive. But here, where the child had no prior presumptive status of being alive, as he was not yet born, you might say that one may not desecrate Shabbat in order to save his life. Therefore, it is necessary for Shmuel to teach us that even here one may desecrate Shabbat for the possibility of saving the fetus’s life.