8 weeks or 6 weeks! Someone help

So, my last period started on 26th June, generally a 28 days cycle. pregnancy test on 23rd July with clear blue said 2-3 weeks so I'm assuming I ovulated around 8th or 9th July. So counting from LMP I'm 8 weeks today, but when they do an US do they bring this down depending on when they think you ovulated or how big the baby is? Because based on my ovulation I'm probably only 6 weeks??? What do I go by? So confused

Thanks for any guidance or experience! X