Low HCG levels at 6 weeks?

yesterday i was 6 weeks pregnant. today i was spotting a bit and my ob told me to go to the ER bc i would need to get the RH negative shot if i miscarry. this is my second child. both planned. They did an ultrasound and they saw the sac, its measuring at 5wks and 2 days. they said that my hcg levels are soooo low at 900 that i will miscarriage in the next few days with lost of blood and cramps. I've had cramps this whole pregnancy but had them with the first as well. They said that it could be very early to hear no heartbeat or movement looking at my ultrasound things could change maybe i am just to early to see whats going on. but the dr on call said that the hgc levels that i will miscarry. has anyone at levels at 900 at 6wks with spotting and still carry to term?? i really don't want to lose this baby. husband just left he is in the army and it would be a long time before we were able to conceive again