Can't find heart beat at 16 weeks using doppler.

Hi, Im 16 weeks pregnant and had an appointment with my Ob today.  She said that we should be able to hear the baby's heart beat by now, so she got the doppler out, but couldn't hear the heart beat.  She said that the uterus feels like it the correct size though.  This has been worrying me - has anyone else has an experience like this??  Is it normal not to be able to hear a heart beat at 16 weeks - if anyone knows, please let me know! thanks