So when I had my ultrasound done last weekend I thought I was 14 weeks and 4 days. But the tech said…

So when I had my ultrasound done last weekend I thought I was 14 weeks and 4 days. But the tech said I was measuring 15 weeks 4 days. I never knew the exact day of my last period because I never kept track of it due to never having a normal period (sometimes I would go 3 months without a period due to my chemo). So my question is, will I be 16 weeks this week or 15? Should I just go according to the size of the baby? Please help.