Measuring small at first ultrasound

Laury • 1 & Done!
Had my first ultrasound yesterday. According to my LMP I should be 8 weeks but I'm measuring at 6 weeks. Dr said sac with yolk is there and she's able to see fetal tube but no heartbeat yet. So now I'm stressed about the baby not growing how it's supposed to. Anybody else measure earlier than expected with positive outcomes?