Apply to extend your visit visa before it runs out, see form at
http://www.cic.gc.ca/english/information/applications/visitor.aspWhile your application for extension is being processed, you have implied status. Even if they refuse to extend it, it will take them long enough that your baby will already be born and you can go back, did everything right and have no problems from immigration in the future. Good luck and congrats on your baby.
By the way, you can see at
http://www.cic.gc.ca/english/information/times/canada/process-in.asp#temp_res that visitor extension permits are right now taking 83 days to process. Should be more than enough time.