I went in at 10 weeks and I am slim and didn't hear it. The doctor said it is typlically ("meaning" most usually,) after 11 or 12 weeks. The doctors say that if your baby is laying really low that also can make it hard to hear the heart beat. My daugther is 8 weeks and when I was pregnant with her you couldn't even tell till I was about 5/6 months, eve though I was small. Most people were shocked to find out, because I was small and carried small.
But, the truth is, I don't use pregnancy as an excuse to over eat. You are only supposed to gain about 25lbs approx. for the whole pregnancy.
ANyways, There was a time at 6 months they had a hard time finding her heartbeat due to the way she was sitting inside me. My daughter was born 8lbs 3 oz, so she wasn't a tiny baby - and she is breast fed and was already 10-lbs at 2 weeks old, so she is a healhty baby. So, it was never due to her being unhealthy or small, just how she'd sit inside me at times.
Good luck - you will probably have to wait till your next appt, but by then you should be able to hear the little heart beat inside you. COngratulations :)