Everything the other ladies said i agree with...after a certain point, they just don't double any more, and u/s is the only way to really know whats going on. You are in that unfortunate waiting time, and I know first hand how crazy that can make you. Dr.'s are not always forthcoming about information, because so many things at this point in your pregnancy can be interpreted both ways, and they just don't know for sure. For example, an irregular shaped sac can be a negative sign, or it can mean nothing at all. Every pregnancy, and every women is different. What is important now is the growth and development between ultrasounds. I am so sorry you are going through this, I have been there, and I know how tough it is. Hang in there, and I hope all turns out well for you.
the higer your levels get, the longer is takes to double. as you get closer to like your 8 and 9th week, it can take up to 4 days to double. i would go back for another ultrasound a week from your last one.
Sorry to hear that your going through this! I know the unkown of this all can just be the worst!!!! as far as the levels go, in early pregnancy they should double 60% in 48 hours, so thats doubling like many people think.. they "should" double or a little more than that in 72 hours though.. the further along you get and higher your numbers are they don't double as fast thats normal.. so i would just ask for another ultrasound, once your at a certain point in your pregnancy an ultrasound will give you WAY more information than a beta... and for me i needed the peace of mind to knwo what the heck is really going on in there.. the wait just killled me! my dr didn't even repeat my betas past the 4.5 week point.. just wanted to go based on ultrasound.... as far as the "irregular" sac.. that can mean a lot of things.. sometimes it can be a sign of collapsing or impending miscarriage, but with a great heartbeat like that, i am praying thats not the case for you.. also many people have what is characterized as "irregular gest sacs" and have completely normal healthy pregnancies.. sooo i think the best thing would be to get a repeat ultrasound, for your peace of mind just to know either way whats going on! Your in my prayers that you have a heathy nine months ahead of you! : ) keep me updated!