Robert Huth and John Terry got the goals as Chelsea kept their dream of a quadruple alive by seeing off Birmingham in the FA Cup fourth round.
Huth, who also hit the bar, got away from his marker to head in Damien Duff's corner after six minutes.
Darren Anderton twice went close but saw a shot blocked by Glen Johnson and another tipped over by Carlo Cudicini.
But Chelsea secured their safe passage when substitute Frank Lampard crossed from the right and Terry headed home.
Premiership leaders Chelsea rung the changes, with Frank Lampard, Geremi, Didier Drogba and Arjen Robben all starting on the bench and Jiri Jarosik handed his first start.
But there was little sign of Chelsea easing off and they were ahead inside six minutes.
Eidur Gudjohnsen flicked the ball on and Marteja Kezman hooked the ball over his head to release Duff on goal.
He never really got the ball fully under control but still managed a right foot shot that needed a fingertip touch from Maik Taylor to prevent it finding the bottom corner.
From the resulting corner Martin Taylor appeared to be blocked by Terry leaving Huth free to powerfully head home.
Birmingham responded well to the early setback with Anderton firing in a powerful shot that was blocked by Johnson's midriff.
But other than that Chelsea dominated the opening 45 minutes and should have added to their early lead.
Duff missed a glorious chance after being released by Joe Cole and also saw a shot deflected wide by substitute Olivier Tebily.
Huth was again left unmarked from a corner and his volley into the ground smacked against the bar.
Birmingham's relief at seeing Duff withdrawn at half-time will have been tempered by the sight of the arrival of Robben off the bench.
But the visitors had chances and went close when Anderton found some room to fire in a shot that Cudicini acrobatically tipped over.
The former Spurs midfielder also had a shout for a penalty when a long-range effort looked to have hit Terry's arm, while Robbie Blake latched onto a weak backpass from Terry only to head straight at Cudicini.
But with 10 minutes to go Lampard's cross found the head of Terry from the edge of the area and he directed it into the bottom corner to seal the tie.
Chelsea boss Jose Mourinho:
"The first half we played very well.
"In the second they improved a lot. They had a different spirit, they were a lot more direct to Emile Heskey and we had a difficult period. "But we had two very good central defenders and I think JT's (John Terry) goal killed the game. I think we deserved the victory."
Birmingham boss Steve Bruce:
"We feel a bit aggrieved. It's a training ground routine. We've all done it but you hope the ref spots it.
"The second half we came into the game and caused them a few problems.
"We've got injuries galore and it's always going to be a tall ask coming here, but there were some positives to come out of it."
Chelsea: Cudicini, Johnson, Huth, Terry, Bridge, Cole, Smertin, Jarosik, Duff (Robben 45), Gudjohnsen (Drogba 65), Kezman (Lampard 60).
Subs Not Used: Pidgeley, Geremi.
Goals: Huth 6, Terry 80.
Birmingham: Maik Taylor, Melchiot, Upson, Martin Taylor (Tebily 25), Clapham, Anderton (Yorke 76), Clemence, Carter, Gray, Blake, Heskey (Morrison 73).
Subs Not Used: Vaesen, Kuqi.
Ref: M Dean (Wirral).