Fearful of miscarriage

Elysia

I went in yesterday for ultrasound and review of bloodwork. HcG levels read 4500 then 4600 2 days later. They couldn't find baby in gestational sac so they ordered an HD sonogram for a better look. Baby was found as well as the heartbeat and measured in track based on conception and ovulation dates. I am so confused about why my levels haven't doubled but the baby grew and dated well. The doctor left before she could even review review my results or answer my concerns. Any thoughts?