I had alot of scans to check for ectopic pregnancy due to previous ectopic and the Ob assured me at 6wks1day he could see a HB but I couldn't (although I trusted him that he could see it) I had one at 5.5weeks and could see the beginning of a sac but no fetal pole or yolk sac so they wouldn't confirm the pregnancy was uterine. I also had one at 4.5weeks and couldn't see anything but a thickened uterine lining expected in early pregnancy. Its amazing what a few days made. I was also trim (BMI about 22) and had a retroverted uterus. All the scans were transvaginal.