Can an ultrasound be wrong

The date of conception can't actually be determined, but the fetal growth usually reflects how far along the mother to be is , so i believe an ultrasound is pretty accurate.

It depends when in the pregnancy it is done. The earlier in pregnancy the more accurate it is. So at 7 weeks it is only going to be inaccurate by 3 or 4 days.

If you have your first U/s at 8 months it is not nearly as accurate as babies vary so much in size.