Hcg- slow rising??

Finally conveived after two years with gonal and iui. Yay! I'm worried now though because of my hcg levels. We are 8 weeks 1 days and my levels continue to rise but the doctor says they are slow rising. Pregnant with twins and they are only 61,000 at 8 weeks. The do rise each week.  Doc says not to worry as ultrasound shows two storm heartbeats at 160bpm and both babies measuring at 8 weeks exactly. Anyone else experiencing this??