Reference for Niddah 1:4
אֵיזוֹ הִיא בְתוּלָה, כֹּל שֶׁלֹּא רָאֲתָה דָם מִיָּמֶיהָ, אַף עַל פִּי שֶׁנְּשׂוּאָה. מְעֻבֶּרֶת, מִשֶּׁיִּוָּדַע עֻבָּרָהּ. מֵנִיקָה, עַד שֶׁתִּגְמֹל אֶת בְּנָהּ. נָתְנָה בְנָהּ לְמֵנִיקָה, גְּמָלַתּוּ אוֹ מֵת, רַבִּי מֵאִיר אוֹמֵר, מְטַמְּאָה מֵעֵת לְעֵת. וַחֲכָמִים אוֹמְרִים, דַּיָּהּ שְׁעָתָהּ:
Who [is considered] a virgin [such that her hour suffices]? Any who has never seen [menstrual] blood all her days, even if she is married. A pregnant woman? [She is considered to be pregnant] once her fetus becomes known [i.e. at the end of her first trimester]. A nursing woman? [She is considered to be nursing] until she weans her child [i.e. for twenty-four months]. If she gave her child to a nursemaid, or weaned him, or if he died [within twenty-four months], Rabbi Meir says: she is rendered impure [retroactively] from that time to that time [on the previous day, i.e. twenty-four hours earlier]. And the Sages say: her hour [of discovering blood] suffices [to render her impure only from that moment].