Ultrasound has me confused

I thought I was ten weeks went for ultrasound and she said baby is measuring 8 weeks… when I commented on me thinking I was ten she said the baby has been there for eight weeks so my period was before that… anyone help clarify what she meant I’m confused

Edit also I know it may not seem important the two week difference but my husband cancer treatments finish may 19 so two weeks makes a huge difference to know what my chances of having baby before he is done chemo as we got pregnant before the cancer diagnosis