Ok so I found this:
Normal hCG levels vary widely between different women and in different pregnancies for the same woman. Be very careful when trying to 'interpret the numbers'. During the first 12 weeks of pregnancy, the level itself is NOT as important, as is, how much it is rising every few days.
Some normal pregnancies will have quite low hCG levels and still progress, ending in the birth of a healthy baby. The best way to confirm if a pregnancy is progressing is to repeat with a blood test in 2 to 3 days time, and perhaps again 2 to 3 days after that. This is aimed at seeing if the hCG level is rising adequately.
and a thread for ppl with low betas https://community.babycenter.com/post/a27708761/hcg_15_dpo_only_18.4