6weeks (LMP), yolk sac but no embryo

Hi. I need your opinions. My LMP is 02/02/11. When i realized i'm one week delayed i checked if i'm pregnant using preg kit. I was positive. On 03/12/11 (5 weeks 3 days), i had my first u/s, it showed an early gestation sac measuring 7.3mm. Doc asked me to come back after two weeks. But i went back to her a week earlier because i'm so excited and worried at the same time. That's today, 03/19/11 (6weeks 3 days). I thought i'll see an embryo with heartbeat. But a yolk sac measuring 2.3mm and mean sac 15.3mm were seen, no fetal pole or anything. It also says in the report that based on measurements, i'm 5wk preg not 6wks. My pregnant friends who had u/s on their 6th week were already seeing embryo with heartbeat at this point. I am worried that the baby will not develop... what do you think about my situation? i want to have a baby but it doesn't seem to be happening...