The nurse said on the phone that "once you have 3 betas doubling every 48 hours you move on to the US - no more betas". Which I wish was not the case. I would prefer to do another to see the number near the time of the ultrasound so I can be mentally prepared. Low beta (which mine are still low) are more often associated with poor outcome or worse, like an ectopic.
Now that I write this I think I am going to call and see if I can get orders for a beta later this week. Can you tell I am still freaking out?
I remember that feeling, too, after my 4th beta when they said, "see you at 6 weeks!" I wanted a beta every week. After my 500 number on day 17, it went to 2,000 on day 20 and then they stopped all betas. The wait was excruciating (and I too worried about an ectopic pregnancy) but at six weeks, the wait was worth it to see that little heart beating. I hope this is the same case for you, and I think you should ask them for one more beta just to make you feel better.