1,090. Hefsek Taharah

158:4 If a woman knows that she is not pregnant and goes to the mikvah, then she miscarries within 40 days, this does not count as a birth. This is because a fetus is not considered formed under 40 days. Therefore, this miscarriage is counted as menstrual uncleanliness. Even if she didn't see blood, there likely was some blood that became lost because the womb simply does not open without some blood.

159:1 If a woman sees blood during her ritually pure days, she must count five days including that day, i.e., the day on which she saw blood plus four more days. Even if she saw blood at the end of the day - even if the community made Shabbos early - if it's still daytime, then this day is included in her count. On the fifth day before twilight, she should check herself thoroughly and wash at least her genital region. She puts on clean white undergarments; the rest of her clothes should also be clean. This is called "hefsek taharah" - concluding in purity. At night, she should put a clean white sheet on her bed. Her pillows and blankets should also be clean. The next day she starts to count seven clean days. There is no difference whether she saw a lot or a little blood; even if she saw one just drop or found just a stain during her ritually pure days of purification, she must wait five days. Similarly, even if she saw blood the whole five days, if it stopped before twilight on the fifth day, she can perform a hefsek taharah immediately after.