Can I be forced to work in a toxic environment while pregnant?

I'm a Chemist in Holliston, MA and I'm also pregnant. My job requires me to come in contact with several chemicals that are known to cause birth defects. I have to climb ladders, and lift heavy loads during the work day. I only get 1 break for lunch, so I have been suffering from dehydration as a consequence. I'm also frequently required to work overtime, which further limits my fluid intake during the day. I have told my employer about my pregnancy and I requested a job re-assignment. My employer told me that I wasn't qualified to do anything else at the company (which isn't true since other employees have been hired with no degree and no relevant work experience). They told me I can either quit or continue working in the same position with no changes. If I get a doctors note, will they be required to change my job duties or will they be able to fire me and if they fire me (they won't list pregnancy as the reason, they will likely say I made some type of mistake as this is their MO) will I be eligible for unemployment?