I would talk to your doctor & tell him/her what your job is and what you do to get an idea of when you should go on mat leave!
I worked up until 2 days before I had my first daughter, but I worked at a gas station and I also work at Lowe's (I work in Paint and Home Decor, well now I work in Customer Service), I would ask your Doctor, a lot of people can work lifting heavy while their pregnant for a while because they did it before they were pregnant... but like I said, I would double check with your Doctor and see what he/she recommends. :)