Hcg level apparently low so Confused hospital called me back tomorrow

So a couple of days ago I posted a few pics of my pregnancy tests all with really really faint lines. Three days in a row they all came out the same not any stronger on any of them (4 tests) I spoke to my gp who gave me a blood test and last night the hospital rang to say my hcg level was "only 79" and that I have to go back on Sunday for another test. I'm aprox 13 days past ovulation I think should this be higher. They said its possible I may miscarry as the level is so low but my thoughts are it's still really early. Anyone know much about this or had this issue and still carried to full term? X