Hcg level not doubling

Kelly
Last Thursday (3w6d) my Hcg was 34, when I found out I was pregnant. On Monday my Hcg level was taken (4w2d) and it was 218. Today my level was 748 (4w6d). Because it's 4 days apart it should be around 850 correct? Does this mean that I am going to miscarry?