Curious?

When do doctors usually change your due date ? 
I'm suppose to be 32 weeks and 2 days but when I went in for my ultrasound on Friday I was 31 weeks and 6 days she told me my baby is measuring up to 33 weeks and a couple days ' when she was doing the ultrasound measurements it would also say 34 weeks so does this mean that my due date she'll change or not ?