Dear Shelly-
Yes, I have heard of these levels being successful. It is a misconception that the levels have to 'double'. Increases of 1.6-1.7 every two days are acceptable, and there have even been successful pregnancies when HCG levels only increase by 50% every two days! Along a 1.67X curve, we would have expected a beta of 690 on June 29 (pretty close to 718), and around 1900 four days after that. While your level of 1600 is a bit lower than that, many reasons could account; for example you may have had an initial implantation of more than one embryo, and one of the original implanters has stopped developing but the other(s) are still doing fine.
The wondering and anxiety are tough; but these numbers are not horrible. Let's keep our fingers crossed. Do you have an ultrasound scheduled for sometime in the near future?
I thought I would post since it seems the docs are out this week and I know what you are going through with hcg's. Mine didn't double either, they went up about 80%, about like what yours look like. As long as they are going up more than like 60%, there's not much to be concerned about. We did IVF, transfered 3 embryos back, last week we went for US, 2 sacs, but this week, unfortunately, it looks like we are going to lose one. Your numbers do look high, so maybe you have twins, and even if you don't, I think your pregnancy will be fine. Good luck to you