No Baby???

I hope this ques don't sound crazy, but I've heard and read that sometimes when a woman have a ultrasound they find no baby but a fetal pole, are no fetal pole and a yolk sac with no baby can anyone help me to understand that please?? I have a ultrasound sch for today and i'm very nervous. I'm about 7wks and 5 days today and hope everything is okay.. Thanks in advance!!!