Has anyone been 3 weeks off on dates?

I’m so confused.

I’ll try to make this short, I had my LMP APRIL6 I had a positive test MAY1, my first ultrasound I should have been about 5 weeks, I went in so early because of cramping, nothing showed on ultrasound except endometrial thickening, which they said is normal for being like 2 weeks pregnant, next ultrasound at 7 weeks showed a gestational sac and yolk sac, 8 weeks also showed only gestational sac and yolk sac, dr said it is a miscarriage and he wanted to fully confirm with one more ultrasound at 9 weeks before scheduling a d&c, well 9 weeks there is a fetal pole, measuring at 6 weeks. He said still it may be a miscarriage unless my dates are way off. Any way I would be that off on dates when I had a positive test May1 and I know for sure when my last period was?