I had an ultrasound when my levels were almost 4000 and they couldn't see anything. I thought I was 5wks2days (I had a history of ectopic pregnancy and was a high risk of a repeat) That was a transvaginal ultrasound also. As I was having scans every few days until they could see if bub was in the right spot I had a repeat at 6wks and they could see a uterine pregnancy and fetal pole. Its pretty common under 7weeks not to find bub.