1st time Pregnant Nurse...how do you reveal to boss

I work on a very physical unit (Rehabilitaion) with head injuries, post CVA's and some orthopedics. At orientation we were told that there were no "light duty" positions. I have been at this position for 6 months. I found out this week that I am pregnant (approx 6 weeks) so I'm not sure how to approach the subject with my boss. I understand the laws that prohibit discrimination with pregnancy but realistically I'm not sure that I can handle this type of position for my entire pregnancy. Does anyone have any ideas of what to say to my boss? Does HR need to know? When do I reveal pregnancy to the rest of my co-workers? I'm not really close to anyone so I don't really care to reveal anything until after my 1st trimester. Is that a bad idea? I work 7p-7a Thurs-Sat. I am also in school during the week getting my prereq's for my RN finished so I am overwhelmed but happy in the same breath. I was recently diagnosed with pseudotumor cerebri (essentially I make too much cerebral spinal fluid) and although after a few months of LP's and problems with my vision everything has cleared up but I must attend to my health closely and hormones can make the condition worse. Do I start hounding the Doc's now to push for disabilty/bedrest? I am not the type of nurse to turn my nose up to hard work and I don't want to mention possible short term disablility to my employer but I'm not sure what else to do? Do they have to offer me some type of "light duty" position? Any ideas or any prior experiences anyone can share, good or bad, would be helpful. Thank you in advance for any help you can give me.