What Juniper said, the doubling rate time starts to stretch out longer the further you go along.

I have a friend who's numbers didn't double in 72 hours (only went up marginally (like 100) and she didn't hold out much hope but she now has a 6 week old to prove it's not always the case. Also depending on what time you have your bloods taken from the first BT to the next one could make a huge difference in the numbers as well - if there was a 2 hours difference either way that could mean a lot in terms of your HCG number.

I hope you can see something on Thursday to put your mind at ease.