Due date changed 5 days

My first ultrasound was at 7 weeks 2 days making my due date 11/17/15 I used clearblue ovulation tests and got my smiley on 2/24 well at my ultrasound they said the baby was measuring 6 weeks 2 days and they changed my due date. Which one would you go by? It's a 5 days difference and my old OB said they wouldn't change the date unless it was 7+ days so idk why my midwife is different but My sister said maybe my baby is just measuring smaller and everyone is different.