Parashas Shemos · Passuk 1:16
וַיֹּ֗אמֶר בְּיַלֶּדְכֶן֙ אֶת־הָֽעִבְרִיּ֔וֹת וּרְאִיתֶ֖ן עַל־הָאׇבְנָ֑יִם אִם־בֵּ֥ן הוּא֙ וַהֲמִתֶּ֣ן אֹת֔וֹ וְאִם־בַּ֥ת הִ֖וא וָחָֽיָה׃
saying, “When you deliver the Hebrew women, look at the birthstool:cbirthstool More precisely, the brick or stone supports used by Egyptian women during childbirth. if it is a boy, kill him; if it is a girl, let her live.”
Torah Temimah points to R’ Chanan — Sotah 11b (verified).
על האבנים — מאי אבנים? א״ר חנן: סימן גדול מסר להן, בשעה שהאשה כורעת לילד יריכותיה מצטננות כאבנים.
English: “On the ovnayim” — what are ovnayim? R’ Chanan said: He transmitted to them a major sign: when a woman crouches to give birth, her thighs become cold like stones.
Custom — technical reading of ovnayim as a birth sign.
Chazal teach that Pharaoh tried to equip the midwives with a concrete physical criterion for identifying the critical moment of delivery.