Despite a resounding 4-0 win over Southend, Blues boss Steve Bruce tried to play down his table-topping side's title credentials.
First-half goals from DJ Campbell and Stephen Clemence plus second-half strikes from Gary McSheffrey and Radhi Jaidi made it 10 wins in 12 matches for City.
Birmingham took the lead after seven minutes when McSheffrey's cross found Campbell 16 yards out at the far post and he slammed the ball past Darryl Flahavan.
City seemed content to soak up early Southend pressure, perhaps hoping their hosts would tire following their mid-week defeat in extra time to Tottenham in the Carling Cup.
They almost paid the price when Marc Gower lofted the ball to Alan McCormack in the 34th minute, but he fired into the side netting when it seemed easier to score.
Three minutes later Nicklas Bendtner's cross into the box was only partially cleared by Efe Sodje and Clemence was on hand to beat Flahavan for his second goal in consecutive games.
Southend saw Freddy Eastwood head narrowly over before McSheffrey was denied by Flahavan as the half ended.
Birmingham maintained the pressure with Jaidi just off target, while Campbell had the ball in the net after referee Mike Thorpe blew for a foul on the edge of the box.
The travelling fans were furious that the advantage had not been played, but it was irrelevant as McSheffrey curled the free-kick round the wall for his 13th goal of the season.
Shrimpers boss Steve Tilson responded just after the hour by bringing Matt Harrold, Simon Francis and Gary Hooper, and the latter nearly got a consolation goal.
But the league leaders rapped things up when Jaidi headed home Damien Johnson's right wing cross from six yards at the far post six minutes from time.
Birmingham manager Steve Bruce played down talk of Championship-winning form but admitted: "We are on course and we have a chance.
"15 players have gone since last season and we have brought in a lot of younger players. It has been a huge turnaround, so we are delighted it has clicked and it can only get better.
"It was a potential banana skin coming here because we are top and they are bottom, so it was a good win".
Southend manager Tilson admitted his side's Carling Cup defeat had taken its toll.
"If we had got a good start then the tired minds and tired legs might have gone away," he said.
"But when you go down within five minutes then the tired legs and minds gets worse.
"We got punished today. Yes, we were tired, but they are a good side"