Not normal hcg?

Kaela

So my ob is monitoring me closely due to my last pregnancy ending in a miscarriage. I got a bfp on the 3rd of April and had labs done: 4/4: hcg was 24, 4/5 hcg 37, 4/9 hcg was 173, 4/16 hcg was 2,636. She said that my number weren't going up in a normal way so I'm getting an ultrasound today. When I did the math myself the numbers are doubling and then a bit more so I'm confused by her cautious comments. Anyone else had this happen?