HELP! Please

 My doctor gave me two different due dates & I have no idea which one to go by! I had my mirena removed December 17th 2015 after a year of having it, then started my period December 18th (the next day). I didn't get my first positive pregnancy test until I was supposed to get my period again (end of January). I know baby will come when he wants to but I just need to know.. Which due date should I go by? FYI THE  BABY STARTED OUT MEASURING A WEEK BEHIND THE DUE DATE I HAD THAT WENT BY MY LAST PERIOD.