The issue of a woman seeing blood once a month is called "Nida". If she sees after 7 days from the start then she is a "Zava".
I had a few thoughts about this today.
One is just that i wanted to share some information that I have had to share with others after it came up already few times, [even though I have nothing new to add to the subject].
The basic way to go to a natural body of water after seeing blood is even with clothing. I mean to say that according to the law of the Torah only רוב ומקפיד הוא חציצה. That is the only time when going into the water can be invalid is if one is wearing clothing that stops the water from getting to most of the body and also that one does not want the water to get to the body. That is for a "division" between one and the water one would mean deep sea diving gear. However there is a decree from the time of the Scribes that also מיעוט ומקפיד או רוב ואינו מקפיד חוצץ that is if there is something like a band-aid that one does not want the water to get through would also be a division between one and the water. Or clothing that would prevent the water from getting in even if one would not care if the water would get in. But there is no decree on a case where מיעוט ואינו מקפיד. Thus if a woman or man would go into the sea or a river with clothing that does not prevent the water from getting in that is a valid immersion. [This is from the first part of tractate Eruvin, but it is also brought in tractate Nida. I have not added anything new here.]
[A nida goes into a natural body of water after seven days even if she saw blood for the entire seven days. (Which anyway almost never happens. A woman usually sees for 3, 4 or five days). The situation with a Zava is more complicated. Let's says she saw on day 8. She is a small zava. Then she waits one day and goes into a natural body of water. Same for two days. But if three days she is a regular Zava and needs seven clean days. That is she needs to wait for an entire period of seven days without seeing any blood and then go to a river or sea and immerse and then she is clean.
[A nida goes into a natural body of water after seven days even if she saw blood for the entire seven days. (Which anyway almost never happens. A woman usually sees for 3, 4 or five days). The situation with a Zava is more complicated. Let's says she saw on day 8. She is a small zava. Then she waits one day and goes into a natural body of water. Same for two days. But if three days she is a regular Zava and needs seven clean days. That is she needs to wait for an entire period of seven days without seeing any blood and then go to a river or sea and immerse and then she is clean.
