Niddah 9:10

If a woman’s period typically started on the fifteenth of the month but then it changed and started on the twentieth, she is prohibited to her husband on both days. If it changed to the twentieth twice, she is prohibited to her husband on both days. If it happened three times, the fifteenth becomes permitted and the twentieth is established as her new fixed time. This is because a fixed time is only established when it happens three times and an existing fixed time is not eliminated until it fails to occur three times.

Niddah 9:11

Hymeneal blood is compared to wine in that it can be red or black; some vines produce a lot and others produce a little. Rabbi Yehuda says that every vine produces wine; one that doesn’t has withered (i.e., menstruation is a necessary part of reproduction).