Hcg not doubled

S

10/2 got a negative hpt. On 10/7 got a very positive hpt. 10/7 hcg was 51UI. 10/9 hcg was 54UI. Does anyone have any hope that i may still double within the next 48 hours? Has this happpened to anyone else? Theyre estimating I'm 4 weeks but not sure since im irregular and i didnt track my cycle last month.