High HCG levels

Those of you who have or are having twins, what were your HCG levels at 5 weeks? I had my levels tests on Saturday and they were 27786 but my ultrasound is only showing me to be about 5 weeks. We couldn't really see anything in the sac. My friends think I could be having identical twins but it's just too early to diagnose. I looks at this chart and it shows I would be AT LEAST 6 weeks but you'd be able to see a baby during an ultrasound if that was the case.