HCG Level

If someone can help me be great. So I went in 4/27 and got a HCG test done around 5pm and it was 12797. I went back in 4/29 around 230 cause I was already out and about so it was only 45.5 hours in between and it raised to 19,328. Dr was testing me cause they were afraid of a miscarriage or blight pregnancy due to the ultra sound thinking the baby was behind on growth (which dr said could be because of my irregular cycle) . I was about 6 weeks and 1 day on 4/27 . So question is did anyone else test not really double when they did HCG test ? I have to wait till may 10 for next ultra sound but sitting in the back of my mind