My earliest ultrasound was at 8 weeks, how accurate would my results be for determining paternity?

Paternity?: The ultrasound at 8 weeks should be able to nail the date of conception down to less than plus or minus a week. As far as nailing down who the male contributor is, that is another matter. Sperm may remain viable for up to 5 days so you need to figure out when you conceived and then with whom and when you were intimate. A dna test would be definitive to answer your question.