I very low betas that didn't rise normally, either. My levels remained abnormally low & it took longer than usual to even confirm a uterine pg. (About 6 wks. At 6 1/2, a uterine pg. with a heartbeat was confirmed, but I was told the sac was not shaped right.)
He is now 2 yrs. old & as happy & healthy as can be. I have no explanation for what I went through, but the result was a healthy baby.