People used to believe that babies born at 7 months were more likely to survive at 7 months than 8 months. A male is born face down wheras a female is born face up. Another source describes 2 channels in the male sex organ; one for urine and one for semen? How did these beliefs arise and how could people have believed things like this if they are demonstrably false? Was it really the case that no one bothered to check? Alternatively, has nature changed? How could we check? I believe these ideas are myths which is why I've posted them here.
The scientific method is only a few centuries old at most. Before that, you need to remember that most people were illiterate, and universities or indiviual researchers were driven (or at least influenced) by religion. Retributions for disagreeing with official religious dogma were severe in the Middle Ages when the power of the church was at its highest. For example, study of human anatomy was strictly forbidden by the church as they saw it as a violation of creation.
Leonardo Da Vinci made some of the first accurate Western anatomical drawings around 1506 at the peril of his life. He was accused of ‘unseemly conduct’ (some sources even say witchcraft) by Rome and ceased his anatomical studies.
As for the specific examples you give in your question:
Premature Birth Viability: The oldest source for this myth is in a 5th Century BCE Greek text attributes to Hippocrates. It also appears in the Talmud and Medieval texts. The origins of this superstition have been researched and detailed in this paper. The origin seems to be the special status of the number seven throughout history. It has a special meaning in Greek mythology, and in the New Testament, it symbolizes the unity of the four corners of the Earth with the Holy Trinity. The number seven is also featured in the Book of Revelation (seven churches, seven angels, seven seals, seven trumpets, and seven stars).
Face Up/Down Births: this superstition originates in Babylonian texts and was copied into the Talmud. The "reasoning" seems to have been: "boys are born face down, so they do not need to see their mother's "shame", while girls are like their mother, so they are not ashamed". See page 128 of this research paper on Birth in Babylonia and the Bible
I couldn't find a research paper that details the origin of the superstition on dual male sex organ channels.
@Alhal as to your question in the commentaries,, that depends on how far you want to go back. The Talmud is considered a key text in Judaism, in which there are two "editions" of the Talmud: The Babylonian (Jewish Babylonian Aramaic) and the Jerusalem (Hebrew) text. If this is as far as you want to go back, then the collective texts of the Tanakh, Tora, Mishnah & Midrash should encompass the belief system of (Monotheistic) Judaism.
However, the origins of Judaism lie in the Bronze Age amidst polytheistic ancient Semitic religions, like Canaanite & Babylonian, of which you can find many remnants in the Judaic texts:
"Who is like unto thee, O Lord, among the gods?" (Exodus 15:11)
Clearly points to this earlier polytheistic & animistic period. The god Ba’al is mentioned about 90 times in the Tanakh. Ba’al was an honorific title of the god Hadad, in much the same way that "Adonai" is an honorific title for Yahweh. The same pattern of association can be found for other gods like El, Mot, Rahab, Moloch, etc. This is called the "pre-Mosaic" (before Mozes) or the Ancient Semitic stage of the religion, and there is much less literature on this phase of what would become Judaism. My recommendation would be to start by looking at Babylonian and Canaanite texts and then derive how much was assimilated into Judaism. Based on your original question, this will probably be more useful, as many of the superstitions you listed originate in the polytheistic period.