The Mishna that closes the ninth perek of Massekhet Nidda waxes poetic in describing vaginal bleeding in a woman. The Mishna teaches:
Women, with regard to the blood that flows when their hymens are ruptured, are like grapevines. One vine may produce red wine while another produces black wine, one vine may yield much wine while another yields little.
In response, the Gemara quotes a baraita where a different metaphor is used to describe a menstrual period:
Rabbi Ḥiyya taught: Just as leaven is good for dough so is menstrual blood good for a woman.
It appears that the Sages of the Mishna took their muse from passages in the Tanakh and in midrashic works. According to the Tanna Kamma a woman is compared to a grapevine based on the verse in Sefer (128:3): “Your wife shall be as a fruitful vine, in the innermost parts of your house; your children like olive plants, round about your table.”
The verse is understood as teaching that if one’s wife is similar to a fruitful vine – one that gives much wine – in that her menstrual flow is strong, she will be able to have many children. Rabbi Ḥiyya, on the other hand, borrowed his material from the midrashic interpretation of the Yosef story, where we find that Yosef’s master trusted him with everything “save the bread which he did eat” (Bereishit 39:6). The “bread” is understood to refer to his wife, as we find when Yosef rebuffs his master’s wife’s advances with the argument that Potiphar trusted him with everything “except you, because you are his wife.” Thus, Rabbi Ḥiyya’s metaphor compares a woman to leaven. Just as leaven makes the dough rise and become bread, similarly menstrual blood ensures pregnancy and childbirth.
The Arukh LaNer suggests that Rabbi Ḥiyya’s metaphor is necessary because it adds an element of clarification to that of the Mishna. The Mishna’s parallel between blood and wine is not perfect inasmuch as the blood is not good in itself, it is only good to the extent that it aids in successful childbirth. This stands in contrast with grapes that are good in-and-of themselves. Rabbi Ḥiyya draws a parallel with leaven, which is not good in itself, but its impact on the dough is a positive one, just like menstrual blood and birth.