yolk sac but no baby

I had a sonogram (at 6 weeks according to ob ) n all they seen was yolk sac. so maybe im not as far as I thought. ik yolk sac visible at 5.5 weeks. I have another ultrasound on Aug 4. I am a wreck cuz I been spotting a lil but my levels are still high. but my ob says just gotta wait til 4 to see how sonogram goes. I Mc on March 18 of this year n im due March 11 of 15. anyone else have this problem with yolk sac n no baby at 5-6 weeks