if you are concerned talk with your ob or midwife. they know if this is safe for you. they know your history and if its safe or not.
also talk to the dentist doing the procedure, they should know the options for a pregnant woman and pick a safe one. if necessary have your ob or midwife talk with the dentist to make sure that you can safely have the root canal.
as far as if it can be completed in one sitting, thats a question for the dentist, every tooth is different so no one but the dentist that has seen it and the xray of it can determin if they can do it in one sitting.