I'm confused with ultrasound dating! so if i'm 23 weeks pregnant on feb 17th, when did i conceive?

A simple estimate: Ultrasound dating is pretty accurate if done around 8-11 weeks of pregnancy. To estimate when conception was, just get a calendar and count backward from the day of the u/s scan. Count back 2 weeks less than the "weeks pregnant" that the u/s estimated. So, if one got an u/s scan on january 1st, and was told she was 12 weeks pregnant, then just count back ten (10) weeks from january 1st!

21 weeks before: This is obviously an estimate but time of conception is generally about 2 weeks less than gestational age.