Huge baby or wrong due date

At my 20 week ultrasound the doctor was concerned that my baby boy is measuring a few weeks ahead, says that is not a normal case that early in pregnancy. All test for any abnormalities have come back negative, so she is now leaning towards my due date being sooner than we originally thought. That last "period" was extremely light and lasted a day, so may have been implantation bleeding and I may have tested too soon to get a positive test result at that time. Having an ultrasound every other check up to watch his growth to try to determine if she is right about a new due date, he is still measuring significantly large. Have any others experienced this, and if so how and when was it officially determined you were due sooner? I am concerned as I am not able to deliver vaginally due to other complications, so will need to agree to a cesarean date soon. Just would love insight from other moms on this to feel more comfortable. Thank you!