BFP but low hCG level.

SF • We have a 5 yo and it took about 4 years of trying for our 2nd, due in July.
AF was due today. After the faintest lines on 3 home urine tests I got a beta hcg test. My doctor said that I'm pregnant but my hcg is lower than she'd like. 13. Retesting on Monday. I'm thinking this will not be a viable pregnancy. Anyone have any positive stories with a level THIS low? I read many posts online and levels in the 50-60's seem low, so 13?!? Thank you!