I'm in a very similar situation with #'s on par with yours. I have an ultrasound tomorrow because the doc wants to see what's going on in there. I'll report back tomorrow as well. Here are my betas over the past week. I used this site here to calculate my doubling times and increase: http://perinatology.com/calculators/betahCG.htm
18 dpt5dt = 963
20 dpt5dt = 1565 (68.52 hours, 62.5% increase)
22 dpt5dt = 2354 (81.5 hours, 50.4%)
25 dpt5dt (3 days later) = 3,995 (94.35 hours, 42.3 %)
So clearly, my doubling time is increasing. This could be on account of the higher overall hcg numbers. After 1500-2000, the doubling times is expected to begin to slowing down, and might be take 72-96 hrs to double.
I have no cramping or bleeding of any sort, and am on progesterone suppositories. I'm estimating I'll be about 5 wks, 6 dys at tomorrow's ultrasound, so they may expect to see at least the yolk and sac, and if lucky, a heartbeat. My non-stop hours of googling and research lead me to believe that this can go either way. Still hopeful, of course, but realistic. This was my first IVF, but 2nd miscarriage (1st from natural pregnancy). We transferred 2 embryos, one of which was a perfect hatching blastocyst. By the way, I was told the same thing - that it could be that 2 embryos latched on, but one is not doing very well. My doc didn't believe it was ectopic, because in his opinion that's less than a 1% chance of happening.
Best of luck! and i'll keep you posted. Please don't despair. Women's bodies are too complex to be reduced to #'s, in my opinion. And I'm an epidemiologist...