nida/zava during pregnancy

Someone emailed me the following:
A small question that I could not find the answer, about the last perek. A few pages discuses the issue of Isa yoledet that saw blood before birth, what is the verdict of the blood if the days were not her days of nida, but in the 11 days of zava. This I don’t understand how a women in her ninth, has days of Nida and zava. Even though if she spotted blood at the beginning of the ninth she is fur sure Tmea, but why is she called also Nida?

This was my response:
I'll do my best to answer the question but let me know if I misunderstood you.

Whenever a woman sees blood she is either a zava, nida, yoledet or
tehora. We know she can only be in one of the last two categories
after she has a baby and those last for a total of 40 or 80 days. So
when she's pregnant and she sees blood (not including during labor)
she has to be either a nida or a zava. The question is which one and
this really leads to a machlokes rishonim. The way most rishonim learn
is the way that Rashi learns throughout the mesechta. Any time that
she sees dam she is a nida unless she is in her yemei ziva. The only
way that she can possibly be in yemei ziva is if she in the 11 days
immediately following her 7 days of nida (it could go beyond the 11 if
she was already a zava but she had to have first seen three straight
days during the 11). So anytime the woman sees dam during pregnancy
she is a nida unless she just completed her yemei nida.

The Rambam argues with Rashi and says that it's a constant calendar.
So it's just 7 days that if she sees dam she'll be a nida then 11 that
she'll be a zava and then 7 of nida and then 11 of ziva and it goes on
forever like that regardless when she actually sees dam. Just to
clarify Rashi a bit more. It starts with 7 of nida when she sees dam
then 11 of ziva and then the 7 doesn't start again until she sees dam.

I hope this answered your questions and/or clarified things a bit. let
me know otherwise.