Worried :(

Laci
According to my last period I should be 8 weeks but had US today and only measured at 5-6. There was no heartbeat present which they told me was normal this early but I can't help freaking out. I'm so scared it's going to end up a missed miscarriage. I've had two healthy children, one ectopic and one miscarriage, anyone else have this and carry the baby full term?