I just received my first ultrasound on January 15th 2013. It said I was about 18 weeks. I have two dates in mind of when I conceived. It was either September 15th or September 29th of 2012. Could you tell me what date is more likely the date I conceived?

Sept 15 to 29 isn't a very big gap. Sorry, can't tell you when you conceived; that is onnly a matter of about 15 days ! Is this a critical question?

Yes it is. I just really need to know which is the more likely date.

To tell you the truth, if we had a crystal ball, could probably tell you, but what you are asking is more or less a magical answer. No one could tell you the exact date of conception. There is always a factor of more or less.