OP - what is the number?
My first beta (at 10 DPO) was only a 7, which barely registers as positive. My doctor told me to expect a miscarriage, even after a second beta showed doubling. It wasn't until we saw a good heartbeat and appropriate growth at 6 weeks that I took a deep breath.
This may be a totally pointless exercise, but here goes (because my beta is the lowest I have ever heard of, so this may be helpful): if I had a beta of 7 at 10 DPO, then at 18 DPO (when you had yours), it would have been 112 if it doubled every 48 hours. That would seem very low to a doctor.
Also, this is helpful:
http://www.betabase.info/chart/basic/single