It totally depends, because every pregnancy is different. Most babies are born 36 to 40 weeks after conception, but they are rarely born exactly on their due date. So there is probably a 4 week window around the dates posted above that could be possible conception dates. If it was a preemie, it was probably conceived even later.
It's much more accurate to go by the Dr.'s measurements early in pregnancy, because they can estimate when a baby was conceived by how large it is.
Possible Dates of Conception: December 21 to December 29, 2007