I'm right there with Heather. If you're 3-4 weeks, 710 sounds like a great number. Has it dropped since your last beta? Is that why your doc is concerned? My first u/s was at 5 weeks & 5 days prior was my second beta, 420. I'm 19 weeks w/twins, so if your 710 was around the same time, I'm confused as to why your doc would prepare you for the worst. Stay strong & best wishes ;)
Are you talking about 3-4 weeks pregnant (from last menstrual period)? If so, then that's only 1-2 weeks after ovulation, and a 710 at that point would actually be very high! Or do you mean 3-4 weeks after ovulation/fertilization (which would be 5-6 weeks pregnant)? If so, it might be a little low, but I wouldn't assume the worst just yet, especially if you're closer to 5 weeks pregnant.
Also, if this is your first blood test, they should do a repeat test 2-3 days later to see if the number doubles. That's the best way to see if a pregnancy is viable, since the number can vary greatly from one person to another. Oh and the cramping can be pretty normal in early pregnancy, unless you've had any bleeding or spotting with the cramps.
After two miscarriages, I would understand if you don't want to get your hopes up (I've had two myself), but I wouldn't give up just yet...
Best of luck with everything! Hopefully you'll prove the doctor wrong and have a happy, healthy baby in about 8 months!