How to calculate

Rikina

So going by my last period I’m 13wks 5days. But at my 8 and 12th week ultrasound I was told the baby is measuring a week early. So do I now go by the week the baby is measure by or continue to count my weeks by my last period? I’m

Confused about that part. They’ve already gave me an earlier due date because of that as well. So should I technically be saying I’m 14wk 5 days? Someone help me understand this