I would contact the ADA and start (secretly) looking for another job. However, they are seriously covering their butts if they want to get rid of you. In some states, it's still legal to fire someone for whatever reason they please and it's hard to prove discrimination. Regardless, reporting them to the ADA will at least put them on their radar and might keep them in check. You also might try contacting NAMI.

In the end, I think they are being ridiculous and downright discriminatory. Even if you weren't Bipolar, you are pregnant for crying out loud! Of course you need as little stress as possible and your brain might be a little cloudy at times.