When I was pregnant, my doc encouraged me to continue doing the type of exercise my body was already used to doing.
As long as your pregnancy is not high risk, you should be able to clean your horse's stall up until the day you give birth. Just take your time, maybe only half fill the wheel barrow during your last month or so.
As far as riding, most docs will not officially ok it, as it is a high risk activity. That being said, I still cantered through my six month, and walked through my eighth.