But if we know that a woman does not actually have her period while pregnant shouldn’t she only be a nidah mdrabbonon if she sees dam?
A woman can be a nida even if she does not actually have her period. Anytime she has bleeding from her uterus she is a nida mdorayso. It is true that a pregnant woman is not expected to get her period (and according to many poskim only after the first trimester), therefore she doesn’t have to automatically be concerned about vestos, (after they pass the first time) because we are not expecting her to see. However if she does see, she is a nida just like any other woman. In fact if she does see while she is pregnant she still have to be concerned as if she has a “veses sheino kavuah” ( an cycle that is not established) , and if she didn’t see in the first expected time she doesn’t have to be concerned any more than that. For example, if she saw on the 5th day of the chodosh Nissan she has to check herself on the 5th of chodesh Iyar, the next month. If she didn’t see then, she doesn’t have to be concerned about the 5th of Sivan, because she is only considered to have a “veses sheino kavuah”.
Y:D 189-33. Taz 48.