Hcg blood test

Ri

My hcg test at 4 weeks 1 day was 230 and 48 hours later it was at 457. So it didn't quite double 3 points short. I'm really worried because my last pregnancy ended at 8 weeks. How common is it not to completely double? My doctor ordered the hcg test 2 more times next week. I'm just really anxious.