If i'm 30 weeks pregnant when did i get pregnant?

Depends on...: It depends on what one is counting. Officially, the pregnancy started 30 weeks ago. That usually means conception was 28 weeks ago (plus or minus one week, depending on when one ovulates). If one had sex very few times, one might know more closely which time led to the conception. For most women, it doesn't matter because they are having sex with only their husbands (with one guy, if unmarried).

28 weeks ago: Pregnancy dating is based on menstrual cycles. The day you conceive you are considered to be 2 weeks along. So at thirty weeks gestation you actually conceived 28 weeks earlier. That should be accurate to within a few days providing your dating is accurate.