When I told my boss I was pregnant, she said oh.  Nothing else.  She did not want me (or my other pregnant colleague) to volunteer for committees.  However, she was accepting of the fact that I had lots of dr's appointments (I was a high risk pregnancy)  and took time off of work for them.  If you want to maintain your level of functioning at work, I would suggest doing your work and volunteering for projects, etc. the same as previously--and cross your fingers....