You probably cannnot be fired for violating weight restrictions for being pregnant, since employers may not discriminate against pregnant women. If the weight restriction has some valid business ground (e.g. you're working for a restaurant or club with certain expectations for  how waitresses look, rather than the local diner; or if based on a greater weight--or being pregnant, which does restrict acitivity in some ways--in some way you could not physically do the job), it may be the case that the employer could reasonably either transfer you (even if only temporarily) to a non-waitressing position if they have one, or possibly require you to take an unpaid leave. The employer is allowed to weight legitimate business considerations in deploying staff and accomodating pregnant staff, and if there is a valid reason you can't do your waitressing job, they don't have to pay you to not be able to work, though again, it's doubtful they could permanently fire you.