definitely tell as soon as possible. they may be able to help find you a comparable position in the company that would be better suited for you. plus they would be better able to understand why you have difficulty doing certain tasks and be better able to assist you. don't be worried about your boss or whoever flipping out at you. pregnancy qualifies as a medical condition and under certain laws you cannot lose your job or be discriminated against in any way because you are pregnant. also, when you do have to go on maternity leave, you are protected under the family and medical leave rights act [i think that's what it's called] whereby you cannot be fired if you take a leave of absence for legitimate family or medical purposes [and maternity leave/care of a newborn child is one of the legitimate medical purposes].
if they act like complete jerks about your being pregnant i would suggest finding a job where they will treat you with respect regardless of whether you're pregnant or not. you don't deserve to be treated like a second class citizen during these wonderful months leading up to birth.
the only reason why i'm saying this is because about a month ago i worked for mcdonald's [where i'd been working since high school] and when they found out i was pregnant i got my hours cut from 40 a week to 4 a week among other things which i won't mention here [people being rude to me].... and i was one of the best workers [was training for management] and everything. now i'm working in a department store and getting treated a lot better. so yeah, you definitely don't need to deal with being treated badly in the workplace.
well... best of luck!! let me know how things go!
-a