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’ Chanina — Sotah 11b (verified).
אם בן הוא וגו׳ — א״ר חנינא: סימן גדול מסר להן, בן פניו למטה, בת פניה למעלה.
English: “If it is a son\ldots” — R’ Chanina said: He transmitted to them a major sign: a male faces downward, a female faces upward.
Custom — additional birth-sign tradition.
Chazal teach that Pharaoh attempted to make the killing possible at the earliest identifiable moment by giving the midwives clear physical markers of sex.