Blood test is more accurate early then an ultrasound, from what I understand, but I waited till 20 weeks so I could see and know for sure, I called out the sex of my baby before the technician even really looked, I saw his penis right away! But my friend did the blood test and was 4 weeks behind me and found out her sex before I did, but I don't think an early ultrasound would be very accurate.
Found out at 12 weeks but thru a blood test i took at 10 weeks