Tying to figure out how far along I am

Melissa
According to my cycle and when j talked to dr to make appt they said I am 6 weeks. On this <a href="https://play.google.com/store/apps/details?id=com.glow.android">glow app</a> they are saying I'm 4 weeks 4 days... Why would there be such a big difference?? So confused?!