The whole of טהרת המשפחה rests on the woman's ne'emanut — « VeSafrah Lah », she counts for herself and the husband relies on her word; the amatla (retracting a statement by explaining it) and its limits; becoming niddah during relations and the ones (Shulchan Aruch, Yoreh De'ah 185 — 4 se'ifim)
אִשָּׁה שֶׁהֻחְזְקָה נִדָּה בִּשְׁכֵנוֹתֶיהָ — בַּעְלָהּ לוֹקֶה עָלֶיהָ מִשּׁוּם נִדָּה. אָמְרָה טְמֵאָה אֲנִי וְחָזְרָה וְאָמְרָה טְהוֹרָה אֲנִי — אֵינָהּ נֶאֱמֶנֶת; וְאִם נָתְנָה אַמַּתְלָא לִדְבָרֶיהָ, כְּגוֹן שֶׁאָמְרָה לֹא טִמְּאָה אֶלָּא מִפְּנֵי קְטָטָה שֶׁהָיְתָה בֵּינֵיהֶם — נֶאֱמֶנֶת.
A woman who has been held to be נדה among her neighbors — her husband is liable on her account as for a נדה. She said « טמאה אני » and then retracted and said « טהורה אני » — she is not believed; but if she gave an amatla for her words, for example by saying that she declared herself impure only because of a קטטה (a quarrel that arose between them) — she is believed.
Shulchan Aruch, Yoreh De'ah 185:3 (excerpt)LEVEL 01
Hebrew text of the 4 se'ifim with a fluent English translation. The ne'emanut (« VeSafrah Lah »), the woman בחזקת טמאה until « טבלתי », the amatla (קטטה, weakness) and becoming niddah during relations, explained with dignity and practical cases of טהרת המשפחה.
📖 StudyLEVEL 02
In-depth pilpul: the יסוד of ne'emanut (« VeSafrah Lah » + חזקת טהרה / שויא אנפשיה); the nature of the amatla and why it does not avail after a מעשה (wearing the בגדי נדות — Taz sk2, תירוץ הרשב״א) nor after 30 days (Beit Shmuel); עד אחד / the חכם is believed באיסורין (and not לממון — PT sk8); « יציאתו הנאה לו כביאתו », the פרישה באבר מת and the teshuvah (שוגג / קושי, ones — Chatam Sofer).
📖 StudyLEVEL 03
Comparative tables (believed / not believed, with or without an amatla, before or after a מעשה, within or beyond 30 days, a חכם who ruled), golden rules, classic pitfalls and cross-references (sim. קפ״ד, קפ״ו, קפ״ז, קצ״ד, קצ״ו); memorization of the 4 se'ifim of ne'emanut.
📖 StudyLEVEL 04
Two parts: the approach of Chabad and the shitah of the Tzemach Tzedek (Daat HaRav), then the halacha lema'asse of the other poskim — Beit Yosef, Rama, Shach, Taz, Sidrei Tahara, Chochmat Adam, Aruch haShulchan, Beit Shmuel (on the amatla), Chatam Sofer, Taharat haBayit, Shiurei Shevet haLevi, Badei haShulchan.
📖 StudyThe whole of טהרת המשפחה rests on the woman's word. The Torah says « וְסָפְרָה לָּהּ » — she counts for herself — and the husband relies on her word (Shach sk1: « וספרה לה לעצמה », if the husband does not know whether the days have passed, he relies on her). The home is therefore not a place of surveillance but of trust. The woman is בחזקת טמאה until she says « טבלתי » (Pitchei Teshuva sk1: even lying beside him, she must say it explicitly). As with all of טהרת המשפחה, the concrete application is settled with your Rav (or a competent Yoetzet).
The amatla is retracting a statement by explaining why it had been said. If a woman said « טמאה אני » and then « טהורה אני », she is no longer believed after כדי דיבור — unless she gives a plausible amatla (she had said it out of weakness, or because of a קטטה, a quarrel). The Rama teaches that מדינא she is believed, and that not believing her is a matter of מדת חסידות. But the amatla has limits: it does not avail after an act — if she was seen wearing the בגדים המיוחדים לימי נדותה, even with an amatla she is not believed — nor beyond 30 days (Beit Shmuel, cited in Pitchei Teshuva). And if a חכם has already ruled that she is lying, the חכם is believed (Taz sk3). The application is decided with your Rav.
This is the grave case of se'if 4, to be presented with sobriety. If the woman says « נטמאתי » during relations, the husband must not withdraw immediately — « יציאתו הנאה לו כביאתו » (his withdrawal is a pleasure to him like the entry). The conduct: he holds completely still until the אבר dies, then withdraws באבר מת, filled with awe and trembling over the transgression that occurred (Rama). If he withdrew בקשוי בשוגג (unaware of the prohibition), the Acharonim speak of a teshuvah (a fast or redemption through צדקה; the woman needs no כפרה). And if the relations took place outside the time of the וסת and she finds blood afterward, it is an ones: neither of them needs כפרה. This case calls for an imperative referral to a Rav; do not decide alone.