Mishnayos Niddah Perek 4 Mishnah 6

A woman who experiences labor pains within eighty days of giving birth to a female, e.g., she conceived during the sixty-six days of purity, or she initially conceived twins and she gave birth to a female and the birth of the second fetus was delayed, all blood that she sees is ritually pure, as she is currently within her days of purity. And this remains the halakha until the child emerges from the womb, at which point she is rendered impure as a woman who gives birth. And Rabbi Eliezer deems ritually impure the blood that occurs due to these labor pains. The Rabbis said to Rabbi Eliezer: And what, if in an instance where the verse was stringent with regard to blood emitted while resting, namely if a pregnant woman emits blood after her days of purity without any labor pains she is rendered impure, the verse was lenient with regard to blood that accompanies the labor pains; then in an instance where the verse was lenient with regard to blood emitted while resting, i.e., during a woman’s days of purity, is it not right that we will be lenient with regard to blood that accompanies the labor pains? Rabbi Eliezer said to them: When deriving a halakha by means of an a fortiori inference, there is a principle that it is sufficient for the conclusion that emerges from an a fortiori inference to be like its source. In other words, the status of blood emitted due to labor pains during her days of purity should not be more lenient than that of blood emitted due to labor pains after her days of purity. Rabbi Eliezer elaborates: Concerning what type of impurity was the verse lenient with regard to a woman who experiences an emission of blood due to labor pains? Concerning the impurity of ziva. But she may still be rendered ritually impure with the impurity of a menstruating woman. So too, if a woman experiences emissions of blood due to labor pains during her days of purity, she is rendered a menstruating woman.

הַמַּקְשָׁה בְתוֹךְ שְׁמוֹנִים שֶׁל נְקֵבָה, כָּל דָּמִים שֶׁהִיא רוֹאָה, טְהוֹרִים, עַד שֶׁיֵּצֵא הַוָּלָד. וְרַבִּי אֱלִיעֶזֶר מְטַמֵּא. אָמְרוּ לוֹ לְרַבִּי אֱלִיעֶזֶר, וּמַה בִּמְקוֹם שֶׁהֶחְמִיר בְּדַם הַשֹּׁפִי, הֵקֵל בְּדַם הַקֹּשִׁי, מְקוֹם שֶׁהֵקֵל בְּדַם הַשֹּׁפִי, אֵינוֹ דִין שֶׁנָּקֵל בְּדַם הַקֹּשִׁי. אָמַר לָהֶן, דַּיּוֹ לַבָּא מִן הַדִּין לִהְיוֹת כַּנִּדּוֹן, מִמַּה הֵקֵל עָלֶיהָ, מִטֻּמְאַת זִיבָה, אֲבָל טְמֵאָה טֻמְאַת נִדָּה: