Well, housing discrimination is illegal so your landlord cannot legally evict you for being pregnant. That would fall under familial status and it would be rental discrimination. You could have a lawsuit
I believe that you will have to disclose if you have another person living with you. Normally on your lease you list the occupants of the home, and you'll need to add your LO, so yes, eventually you will have to disclose.
While there are plenty of perfectly legal reasons your landlord could evict you, your familial status is not one of them. If you get an eviction notice, file a complaint with the department of housing and urban development immediately. If you Google housing discrimination you should get plenty of info. If you have good rental history and have not broken your lease in any way your landlord must honor your lease. I would definitely put up a fight. The kind of person who would evict a pregnant woman is not the type of person who should be allowed to be a landlord.