what should I go off of?

I found out yesterday I'm pregnant. I was suppose to start and didn't. My last period was the 3-8. Then I conceived around the 17th. So doctor says I'm a little over 4 weeks(my appt isn't for another two weeks she just said on the phone) so I know that they go from when you last had your period. So I'm techinally two weeks. But my apps keeps showing what at 4 weeks is like. So which one should I look more at 2 or 4 weeks?