do you think I will miscarry?

So I thought based on last missed period I was 7 weeks but based on ultrasound I only measured 5 weeks, only a sac seen. I got a BFP on December 27th but it was very faint. I also had my hcg level checked 12 days after BFP and it was 3900 so my question is do you think I'm going to have a miscarriage or is it possible to miscalculate my date based off last missed period? I've had no cramping or bleeding.