Hi H. - working mom issues are never easy! I agree with doing all you can, eating healthy, water, feet up, to make sure you have as much energy as possible. I also agree that you should schedule an appt with the office manager and go in with options/plan for maternity leave. I'll go part time and we can hire a pt temp, that way the pt temp can go ft once I'm on leave, etc. If you go in defensive, "these are my rights" "you have to", etc they will automatically get defensive and the working environment will probably become even less pleasant. Go in first with options, as understand you still have responsibilities as an employee and they have an office to run, not to say that a good employer knows the value of a trained employee and should be flexible. If that doesn't work then get the drs note. If you plan to go back after the baby you want to make sure your pregnancy was on good terms so that your maternity leave and when you come back is a better working relationship.
This "advice" comes from personal experience. I'm a manager who's assistant just came back from maternity leave, and I'm also 18 wks prego with twins and a 3yo at home. There's a balance between getting the job done and being flexible for the most important job - mommyhood :)
Best of luck!