How Can USG Scans Showing Increased Gestational Sac Growth Along With An Increase In The HCG Level Be Identified As Normal?

Im in the 6th week of pregnancy(according to the hcg lab results). I ve had two ustrasound checks so far. My first one showed apx 5mm gestational sac. Repeated after 2 weeks- showed 9.5mm gestational sac. I was confused as no obryo could be seen, I decided to have hcg repeated twice. My first result was 25 000 mlU, and repeated after 3 days- just 28 000. I wonder if this is allright, is this growth rate ok? Seems very slow to some studies as Ive heard. This is my first pregnancy, Im 30 years old. Practically, losing my mind since I got my second hcg.