What is written before this matter? “If a woman conceives and bears a male child” (Leviticus 12:2). What is written after it? “A man, if he will have on the skin of his flesh.”
What does one have to do with the other? Rabbi Tanḥum ben Rabbi Ḥanilai said: This is analogous to a female donkey that became ill and was cauterized and its foal emerged burned. What caused the foal to emerge burned? It is because its mother was cauterized.
Likewise, what caused the offspring to be a leper? It is because its mother did not observe the days of her menstruation. Rabbi Avin said: This is analogous to a vegetable garden into which a spring flows. As long as the spring continues to flow into it in excess, it forms marshes.
So too, one who consorts with his menstruant wife produces leprous offspring. Rabbi Avin read in his regard: “The fathers ate unripe grapes, and the children’s teeth will be damaged” (Jeremiah 31:29). They will read regarding their fathers: “Our fathers have sinned and are no more, and we suffer their sins” (Lamentations 5:7).