Hgc levels - nervous

My hgc levels were pretty low and slow to rise. I was 45 at 3 weeks 6 days, and 163 4 days later. My dr said it's not terrible, but not great, so I'm going back today (a week since my last test) and we need to see a big jump. This is my hopefully 4th baby and I never had this issue with my previous pregnancies, then again I was never tested this early. I'm nervous. Wish me luck!