At the moment, I plan to work up until about 4 or 5 weeks before the due date. This will probably depend on how I am feeling at the time. Baby is due march, and Dec/Jan will be slow and boring at work for me, so hopefully not busy and stressful.
My boss does not seem to be interested or concerned with when I take leave. I might change my mind closer to the date. I would really like to work as long as possible as I do not have any paid maternity leave available. Also, I think it depends on what your job actually is. I work in an adminstrative job, so I sit on my bum most of the day so the work is not strenuous or anything like that, and I get someone else to do the stuff I'm too lazy to do (filing, packing and moving archive boxes etc)