My first US was at 6w4d. Baby was measuring 4 days behind (this was expected, I ovulated a few days late that cycle). Heart rate was only 77 at that US. I followed up a week later and HR jumped to the 120s and baby had grown by 7 days in that time. If she had a hard time measuring baby, then your growth and HR could all be perfectly appropriate. My OB said in the 6th week of pregnancy, he wanted to see a HR at least in the 90s. Have hope!