Hello there,
I just wanted to drop my two cents because I completely understand what you're going through. Last time I was pregnant, I was hospitalized at 5 weeks and 3 days. My beta was 18000 that day; the ER doctor was convinced that, with a beta that high, we would see a heartbeat. We didn't. He told me I was miscarrying. At the same time, my fertility doctor said it was pretty useless to do a beta every day, because even if the baby was dead, the "placenta" would still produce HCG for a little while and so the beta could keep increasing all the same.
Two days later, at 5 weeks and 5 days, we found the heartbeat. But it took the hospital's specialist, a real big-shot of radiology, to find it. Most radiologists wouldn't have found it. Two days after that, at 6 weeks, we found it again and both my DH and I could clearly see the little heart beating on the monitor. My best friend, who is pregnant and also a doctor, went for an early ultrasound at 6 weeks and 3 days. They found the heartbeat, but she wasn't even able to see it; only the doctor did. And her beta was sky high; at first we even suspected she was expecting twins. One of the nurses at my fertility clinic told me it's not rare at all that someone has to wait until after the 7 weeks mark to see the heartbeat.
Bottom line is: don't despair sweetie. I understand you really want to be told that the heartbeat is visible (because then your miscarriage risks drop to 2-3%, which is NICE to hear!!!), and don't we want that ASAP after that big BFP moment!

But betas and how far along you are don't always guarantee you'll see a heartbeat on the monitor. Sometimes it depends on the doctor's ability to detect it. Sometimes you just have a late implanter, or embryos that were a bit younger when they were transferred. Sometimes you just have to be patient and give it a few days (and it's hard!!!). It's impossible not to react to that kind of event (no hb with the first ultrasound), but the game is FAR from being over dear.
Hang in there! *HUGS*
Sophie xxox