If I don't remember my last period and my ultra sound says the fetus age is 5 weeks compared too other fetuses... How long ago did I conceive?

+/- 5 days. Ultrasounds done in the first trimester of pregnancy are accurate to +/- 5 days. So to answer your question you conceived somewhere between 5 weeks and 5days ago to 4 weeks and 2 days ago. Hope that helps!