Niddah 31a writes (Davidson Edition translation):
אמר רבי יצחק אמר רבי אמי אשה מזרעת תחילה יולדת זכר איש מזריע תחילה יולדת נקבה שנאמר (ויקרא יב, ב) אשה כי תזריע וילדה זכר
Rabbi Yitzḥak says that Rabbi Ami says: The sex of a fetus is determined at the moment of conception. If the woman emits seed first, she gives birth to a male, and if the man emits seed first, she gives birth to a female, as it is stated: “If a woman bears seed and gives birth to a male” (Leviticus 12:2).
Was this simply something that the Rabbis derived from the above verse, or was it also a common belief at the time? Is it found in contemporary or earlier "scientific" works?
This would have ramifications on what they might mean by this statement (and perhaps how it should be viewed from the perspective of believing it).