Is she pregnant?

I'm wondering if she should be 8 weeks now, shouldn't you see a baby on the ultrasound? Or could she have only been two weeks when she found out she was pregnant, wouldn't a baby show on a ultrasound at 6-7 weeks? She says she's pregnant, she's got the symptoms. I told her to schedule another ultrasound in 2-3 weeks to see if there's a baby. She said she doesn't need one because she knows she's pregnant. I told her that sometimes you can have pregnancy symptoms and not be pregnant. She still doesn't want one.  Could she still be pregnant or could this be possibly a blighted ovum? 

Any answers will be appreciated. 

Thanks a bunch.