Niddah 4:7-5:1

Niddah 4:7

All eleven days (between niddah states) a woman is in a presumed state of ritual cleanliness. If she didn’t examine herself – whether accidentally, or she was prevented, or she just chose not to – she is ritually clean. If the established time for her period arrived and she didn’t check herself, then she is ritually unclean. Rabbi Meir says that if she was hiding from danger when the established time for her period arrived and she didn’t check herself, then she is ritually clean because her fear holds back her period. When it comes to a zav or a zavah (i.e., their seven clean days) or a woman who is counting day for day (in her zivah days), they are in a presumed state of ritual uncleanliness.

Niddah 5:1

If a child is born via C-section, its mother need not observe the days of ritual impurity and purity, nor must she bring an offering. Rabbi Shimon says a C-section is like any other child (and therefore she must observe these things). All women are rendered niddah when blood travels from the uterus to the vagina (even without coming out) as per Leviticus 15:19, “A flow of blood in her body.” A zav and a man who has a seminal emission aren’t rendered unclean until the source of their impurity comes out.