Due date wrong?

So I went into preterm labor last Thursday. (30 weeks) I was bleeding and having contractions every 2-3 minutes. They gave me a shot and I'm on procardia every 6 hours. Thankfully the consistent contractions have stopped. I'm on strict bed rest until further notice, most likely until my due date. My question is, on every one of my ultrasounds my baby has measured 10 days ahead, except on the very first ultrasound I had to figure out my due date since my periods were so off. I know earlier ultrasounds are more accurate, but I've had like 8 ultrasounds from 16 weeks up and he's measured 10 days ahead, which I think is actually more accurate for conception. What are the chances that I'm 10 days ahead? I would rather be ahead in case he comes early!