ultrasound is done at 6 to 7 weeks and a heartbeat is not detected

I am pregnancy.  My two fetus has detected 87, 96 bpm last week. I had an ultrasound on today. The doctor said that I was 6 weeks and 4 day according to measurements. He could not detect a heartbeat. The size of the baby has grown since last week's ultrasound.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Is there problem?  I’m am worry to them. Please help