I reduced from 5 to 3 day weeks from 32 weeks and finished completely at 35 weeks.
It was a long hr+ drive each way to work and sitting in the car really hurts my back ATM. I just can't seem to get comfy.
As well as having DS, school commitments, weekends full of cleaning, washing, cooking etc it was all getting too much for me
I'm glad I finished when I did. I think I struck a nice balance in that I feel well enough to be pottering about the house in preparation for babies arrival but not too soon that I will get bored.
I would discuss it with your employer and mention your concerns for not leaving them understaffed in a busy period but also looking after yourself.
It's near impossible to know how you will feel until you get closer. Like other posters have said. Maybe tell them a date but also you have the option of asking your doctor to give you a cert. if its not working for you or you have health issues you couldn't have foreseen.