How far off are ultrasounds?

Marissa • Due February 25th. 👶🏻 Married to the love of my life 😍😍
According to <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a>, i'm 7 weeks 1 day. Since my last period was may 24th. Will that be what the ultrasound is? Or can I be 5 weeks, 9 weeks, etc? 
Just curious!