Confused on what dates to go by..

I had an emergency scan on Monday and Baby measured 7 weeks so I've been counting from then since. At my first midwife appointment today she said I'm 8 weeks today based on my last period but based on what the baby is measuring I'm 7 weeks 4 days today. I told her this but she carried on saying 8 weeks. Do they just go off your last period anyway? There's only a 3 day difference but which one should I go by?