Due date wrong??

Ok so I had my lmp on 6/5/15. Took pg test on 7/3/15 and got positive. Took four more all positive. Due date is 3/11/16 according to standard lmp calculation. Doc changed it to 3/20 based off second u/s (not first!). Have c/s scheduled for 3/14 which is 40+3 of the original due date. I have been in L&D twice for PTL, had progesterone shots for 6 weeks also. They had my first child's due date wrong by 6 days too. I've been having lots of false labor contractions and they've been getting increasingly stronger. I'm just wondering what could happen if they make me go to 40+3 waiting on a c/s....  Will it turn into ER c/s?