What Causes Lack Of Fetal Cardiac Activity During 6th Week Of Pregnancy?

Question : Hi I was just diagnosed with a missed miscarriage around 7 weeks(date not 100%) I had a tiny bit of spotting a week after a positive pregnancy test. Went for an ultrasound a week later and was told it was 6weeks3days with no heartbeat. Could not be seen on abdominal us so had vaginal. It looked like a small grain of rice at the bottom of the sac, with the yolk at the top. I went back a week later had a different technician and ultrasound machine. and was told it was 6weeks 4 days no heartbeat. The baby could be seen on the abdominal us now and looked like a baby attached to the yolk in the vaginal. The change in the embryo looked significant for one day of growth. Is it possible the first ultrasound was measured wrong? My doctor is set in the diagnosis and has decided that I will miscarry naturally. I'm currently trying to get in for a second opinion and if it is a miscarriage I do not want to miscarry naturally. I was also told I have a 17mm fibroid and a forward tilted uterus. Any info or opinions would be appreciated as I'm getting none from my doctor. It's my first pregnancy and I'm 36.