Mishnah Niddah 4:5: How far [from birth] must her difficulty be [for her pains to be considered a difficult labor]? Rabbi Meir says: even forty or fifty days [before the birth]. Rabbi Yehuda says: [being in] the [ninth] month is sufficient for her [to consider her pains to be caused by difficult labor]. Rabbi Yose and Rabbi Shimon say: difficult labor is not for more than two weeks [before birth].

כַּמָּה הוּא קִשּׁוּיָהּ. רַבִּי מֵאִיר אוֹמֵר, אֲפִלּוּ אַרְבָּעִים וַחֲמִשִּׁים יוֹם. רַבִּי יְהוּדָה אוֹמֵר, דַּיָּהּ חָדְשָׁהּ. רַבִּי יוֹסֵי וְרַבִּי שִׁמְעוֹן אוֹמְרִים, אֵין קִשּׁוּי יוֹתֵר מִשְּׁתֵּי שַׁבָּתוֹת:

Mishnah Niddah 4:6: A woman who was in difficult labor during the eighty days of [purity following the fourteen days of impurity after giving birth to] a female, all blood that she sees is pure, until the fetus emerges. And Rabbi Eliezer considers them impure. They said to Rabbi Eliezer, "Just as in a case where one is stringent regarding blood when pain has ceased [i.e. that such blood renders the woman who gives birth impure] there is a leniency regarding blood during difficult labor [i.e. which does not render her impure], in a case where one is lenient regarding blood when pain has ceased [i.e. during the days of postpartum purity, which does not render her impure], does it not follow logically that we should be lenient regarding blood during difficult labor [that it, too, should not render her impure]?!" He said to them, "It is sufficient for the case inferred to be treated in the same manner as the one from which it is inferred! Regarding what were they lenient with her? Regarding the impurity of zivah. But she [who was in difficult labor during her postpartum days of purity, if she saw blood during her days of niddah, and not her days of zivah, she] is impure with the impurity of a niddah.

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: