i had an empty sac at 6 weeks. during week 7 baby is said to double in size and by 7 weeks 5 days i saw baby and heartbeat on a scan. everyone is different, do not be put off by people who have seen baby by 6 weeks. chances are everything is just fine :)
Its amazing the difference a week can make as well. if your only 4 or 5 weeks he wont be able to see a heartbeat. just wait a week or two and go back
at 6 weeks yes the doctor should see something or your dates could be off. I had a blighted ovum and my doctor waited 3 1/2 weeks after my first appt. which i shouldve been 9 weeks and still couldnt see anything. But your dates could be off so i would wait another week or two then ask for another sono and go from there. I had a miscarriage in april of 08' and i found out i was pregnant the 5th of june and when i went to the doctor i was so scared because i thought the baby didnt develop again. it turns out i was only like 2 or 3 weeks pregnant. so i went back 3 wks later got a baby and a heartbeat. hes now a bubbly 2 year old that thinks its ok to sneak in mommy and daddys bed every night lol. i would wait dont panic yet it could be your to earlier to see the little guy.