Late ovulation?? Not as far along as should be?

Hi there, does anyone have experience with late ovulation? I should be 6 weeks 3 days according to my lmp, but had an early ultrasound today and only saw a gestational sac measuring in the late 4th week.  They said it could be a miscarriage or it could be that I ovulated later than the normal calculations they go off of and I'm just not as far along.  My guess is miscarriage, but wanted to hear if anyone else had a similar situation.  I go back Monday for another ultrasound to see if there is any progress or if it is going to be ruled a miscarriage.