A late Jon Parkin winner moved Preston into the play-off zone as Birmingham lost ground on leaders Wolves.
Second-placed Birmingham are now six points off the top spot after Ross Wallace fed Parkin, who struck the ball into the bottom corner from six yards.
The visitors had enjoyed the better of an uninspiring game.
Birmingham came close to breaking the deadlock on the hour when Kevin Phillips headed straight at Preston keeper Andy Lonergan from six yards.
Birmingham manager Alex McLeish:
"The disappointing thing was not getting at least a point.
"It's half the season gone and it would have been nice to finish on 50 points, but we knew it was going to be tough.
"We have got to do better in the second half of the season. It was disappointing to lose at the end."
Preston manager Allan Irvine:
"We are showing the kind of form which we did at the start of the season.
"I don't think the win was a present. I think we had to work extremely hard for it and play really well.
"It was a very tight game. Birmingham had their moments, but I think overall we probably just shaded it."
Preston: Lonergan, Nolan, Mawene, St. Ledger, Davidson, Sedgwick, McKenna, Chaplow (Nicholson 80), Wallace, Chris Brown (Parkin 78), Elliott (Hawley 83).
Subs Not Used: Neal, Carter.
Goals: Parkin 90.
Birmingham: Maik Taylor, Hunt, Ridgewell, Jaidi, Queudrue (Parnaby 63), Owusu-Abeyie (Agustien 90), Quashie, Carsley, Murphy, Phillips, Bent (Jerome 59).
Subs Not Used: Doyle, Nafti.
Booked: Ridgewell, Queudrue, Quashie.
Att: 10,943
Ref: Clive Oliver (Northumberland).
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