Update - Doctor said heartbeat was 75. This was 6 weeks 1 day from lmp. HCG levels rose about 4000 in 48 hours. I go for another ultrasound Monday, but am so scared I'm losing this baby. Anybody with a success story after heartbeat that low?

So I went to the ER today (at doctor's request) due to some groin pain and mild cramping. The doctor said blood work showed 6800 hcg and the ultrasound showed a baby in the uterus (they were nervous of an ectopic pregnancy). My OB called me later today after I left and said that the heartbeat was not as strong as they like, but they didn't tell me what it was. This is my first pregnancy and I'm terrified of it ending in a MC. Anybody had anything similar and ended up with a healthy baby? I go back a week from today for another ultrasound to make sure the baby is growing and check on the heartbeat.