Change of Date

I went for a follow-up ultrasound and the MD thought the baby was measuring small.  When I went for my follow-up OB appointment, she thought we should be using the due date from my very first ultrasound...7/19.  Which means the baby is measuring fine.  It was a little frustrating, though, as I was 27 weeks pregnant for two whole weeks!  In the end I know the baby will come when the baby was probably originally going to come, however, my bets are on 7/15.  I am not sure why the date is coming to mind, but most likely it is because it is a Friday in July, and bound to be busy.