I didn't hear the heartbeat until 15 weeks (tried at 9 weeks and 13 weeks), with the Doppler on the stomach. And then he ran from the Doppler (we only heard it a few seconds), and every time after that, he would try to avoid the u/s noise [you can't convince me he didn't "hear" it or sense it somehow!].
If it's a vaginal/visual u/s, I'd be a little more worried, but would still wait. I just read a story of a woman with no heartbeat early in pregnancy, she was sent home to wait for a miscarriage, no miscarriage ensued, doctors recommended a D&C for "missed miscarriage", had another u/s and there was a heartbeat! Her little "miracle".
But if he didn't hear it from the on-the-stomach u/s, I wouldn't worry... rather, I would try *not* to worry (believe me, I worried a little every day after 9 weeks, though I tried not to, and worried constantly from 13 to 15 weeks! then we heard the heartbeat), because the baby is so small that it can be hard to find in your uterus.