Well your baby has been born so a paternity test would be a fantastic idea.
Sounds like you conceived in Feb/Mar. You can use a "retro pregnancy calculator" to determine when you possibly conceived. You can find one at the following website:
Click on Pregnancy Calculator from the menu on the left-hand side. Then choose Retro Calculator.
Here, I did it for you based on your due date (assuming it was the birthday or close to it):
This person was born on Monday, December 10, 2007. This person was conceived on or about Monday, March 19, 2007.
I think the only safe way is a paternity test.. Conceptions dates, like due dates, are estimated,,,
I am due on December 16th and conceived around the 20th of March. If you had conceived in January or February your due date would be earlier
but if in doubt, paternity it out