Hcg levels are rising but not doubling.

I was spotting for 23 days. Out of those 23 days I was spotting really heavy for 2. I cramped really bad for 4 days. Like contractions. I just knew I was having a miscarriage. I could not be told other wise. I went to the E.R. And was told I was having a viable (good) pregnancy but i might have a threathned miscarriage. So with that being said I was very unhappy.  My hcg level came back as 500. Two days later it was 1000. Four days after that it was 1406. Now Two days later it was 1746. I am very thankful and glad that they are rising but there not doubling. I went to the doctor when my hcg results were back at 1000 and we couldnt see anything. But despite all the spotting and cramping my hcg levels are rising.  What should I expect next?