The Shulchan Aruch (and the Mishnah Berurah) when discussing the blessing of Asher Yatzar, (recited after washing the hands after using the bathroom) makes a point that the mouth is closed in the womb and only opens when the baby is born. [Sidestepping for a moment the little scientist in me who would point out this is not a scientifically sound claim.] If the mouth would not open when the baby was born, we would die, they go on to explain.[This is close to some scientific truths about the reversal of blood flow in parts of the body and the closing and opening of certain valves at the time of birth, but I said we're ignoring that for now.]
In taking these two ideas together there's an interesting parallel that may or may not bear fruit:
In Yaakov's encounter with Yitzhak, we learn, "The voice is Yaakov's, while the hands are Esav's." In a number of places, Chazal equate Yaakov with the Godly soul, and Esav with the Yetzer Hara.
From this we see, perhaps, that the only thing that prevents the Yetzer Hara from killing a body outright upon its birth is that its mouth opens. (and the baby cries) The cry, the voice, is the power of Yaakov, and even though, until physical maturity we don't have a proper Godly soul, we do have the imprint of one. That shrill wordless voice of the baby when it is born is a portent of the Torah that will flow from that mouth. For that reason alone, theYetzer Hara is weakened and prevented from taking the child outright.
Were this not enough we see a number of examples in Chazal of those people who were so Holy and so involved in Torah that the Angel of Death (synonymous with the Yetzer Hara) had no power over them. The expression used is that "their mouths never ceased to utter Torah." We find it by David HaMelech, by Rav Yehudah HaNassi and others.
We know Torah is life, that's stated a number of times in the Torah, but perhaps from here we see that the mouth is the sole weapon against the Yetzer Hara.