Doctors changed my due date..?

Ambur
Ok so my husband and I had been trying for about six months to conceive before I finally did. I have always had a spot on, extremely regular cycle. My period always came the EXACT day it was due every month and then obviously since we had been trying for 6 months, I knew EXACTLY when my LMP was...but after my first ultrasounds which was at about 6 or 7 weeks I believe they pushed my due date back a whole week! My LMP was April 6 which means my due date should be January 11th...but the doctor changed it to January 17th...please give me your opinions as to which one I should follow!? I also have a 4 year old that measured small the whole pregnancy and was born at 37 weeks 5 pounds 14 ounces so I think I just make small babies.