Due date determined by ovulation date?

I found out last Friday that I'm pregnant- naturally. I say this because we were using a fertility clinic and IUI prior. I became pregnant in April this year with IUI but sadly it ended in miscarriage. I was waiting for my period to start this month in order to start Clomid again and then go in to do another IUI. Instead I find out I'm pregnant. Now my last period was July 6-12 and I ovulated July 27th (yes, long cycles - ranging from 31-35 days). I asked for an ultrasound due to the previous miscarriage (I am terrified) and the doctor said I was measuring small for what I should be if calculated by LMP, but was exactly where I should be if calculated by my ovulation date. I mean - this makes sense, but is it common practice?