Steve Bruce's side proved a tough nut to crack and managed to carve out a chance at the start of each half at the Lane - Paul Robinson needed top-drawer saves to keep out Jiri Jarosik and Julian Gray before we finally got our noses in front.
The crucial breakthrough arrived after 57 minutes when Matthew Upson hauled down Robbie Keane in the box and the Irish striker picked himself up and slotted the penalty home.
Muzzy Izzet was then sent off for a second bookable offence before Jermain Defoe sealed victory with a stunning strike in added time.
"It was a very tough game but the pleasing aspect from our point of view is that in a game where we know we can play better we've still come out winning.
It would be fair to say we weren't at our best but I wouldn't want to take anything away from Birmingham
"Certainly what we're finding is that there are no easy games, we expected a difficult game from Birmingham - they made a number of changes from their midweek game and came here looking to get something.
"They came with a system, made it difficult for us and got a lot of bodies behind the ball at times. That being the case, it was hard to break them down and it was a relief once the goal went in."
As for the penalty decision, Chris added: "I haven't had the chance to see it again yet but sometimes you just look for first reactions and we had enough players around the incident that thought it was a penalty."
Chris Hughton conceded we weren't at our best and was therefore delighted that we still grabbed the three points against Birmingham on Boxing Day.