Oops! I meant to update this thread!
My second beta was taken ~55 hours after the first and it had tripled, to 93.4.
We had another ultrasound this morning, but couldn't see anything yet. We're hoping that I just ovulated a few days later than we thought, explaining the lower numbers.
I am a little bit worried that I may have miscarried over the weekend though. I had spotting on Saturday and Sunday and it was heavier than the previous spotting. Saturday evening I also had one very sharp stabbing pain.
We did another beta this morning and I'll get the results in about 2 hours. Hopefully it's positive news, but this weekend definitely has me scared.