Ok so I started my period Jun 17 2011 and I had sex on Jun 30th with an ex. Me and my boyfriend had sex from July 12- July 17. I went to the hospital July 18 and took a blood pregnancy test. It said negative. I went back to the hospital August 18 and they gave me a ultrasound and told me I was 6wks and 2days. So when did I most likely conceive

Either the 30th or the 12th are the most likely days, Conception dates are best guess and there is no way to know for sure. Ultrasounds just use size and development of the child based on averages, but there are plus and minus to averages.

So the only way to know who the father is, will be to wait till the child is born and have a DNA test. Also things like who the child looks like has little bearings on it.