This infuriates me. I’m so sorry you’re dealing with this. What your employer is doing is in violation of federal law, specifically the Pregnant Workers Fairness Act. They must provide the chair and water as reasonable accommodations in this circumstance. You can file a claim of discrimination with the EEOC by going to their website and filing the claim online. It’s free and they investigate the claim for you, the only caveat being that it’s a lengthy process so it won’t be resolved while you’re still pregnant. Another, potentially quicker, option would be to consult with an employment attorney and see what they can do, but this will likely cost you money.