Hi, Rose, by the info you have given it does not sound that likely. Could you give a little more information? When was your ultrasound (by calendar date), and what estimated due date did they give you from the ultrasound data?
Based on everything you have said, it doesn't sound like you conceived from the first guy. If you want, figure back to the last time you had sex with him and add 5-6 days to cover the long life of sperm; that would still be no later than about December 15, and then add a few days for the margin for error given that the ultrasound was in your 9th week. Even with all of that added, it doesn't stretch to the 27th. Get a DNA test when the baby is born just to confirm, and then forget it forever.