I had an ultrasound done and was told i concieved on oct.20 if i had sex on oct.30 is there any chance that he could be the father?

Depends: When did you have your first sono? If before 12 weeks then the dates assigned to you should be accurate enough to rule out the oct 30 contact as being the father. If the 1st sono was just recently done there could be enough uncertainty surrounding the date estimate to make the oct 30 contact possible.

Yes, it is possible: Us does not tell you the date you conceived but rather the estimated age of the fetus (pregnancy). Of course, you can figure backwards from the age of the fetus the expected date of conception. However, the us age is only an estimate. There is some variabiltiy so it is conceivable that the uds could be off by 10 days.