Both sides went into the hat for the fourth round, following a 0-0 draw at the City Ground.
But it was Billy Davies' Forest who will have felt they should have claimed victory after seeing Joe Hart make a number of smart saves and Robert Earnshaw miss from the penalty spot.
Forest keeper Lee Camp also made a number of saves to ensure Birmingham didn't make the home side pay - although in the end both sides retained their unbeaten records, with Forest not having tasted defeat in 17 matches and Birmingham 12.
The Reds have been full of confidence at the City Ground in recent months and that showed as they took the game to Birmingham in the opening exchanges.
Dexter Blackstock flashed a shot into the side-netting from a tight angle after the Birmingham defence was split by a pass from Chris Cohen.
Earnshaw tested Hart with a rasping shot on the turn from the edge of the box that had the keeper plunging low to his left to hold well.
A fine, looping ball from Chris Gunter then sent Cohen angling into the box from the right, but his cut back across the face of goal narrowly eluded Earnshaw and, as Birmingham broke, Camp had to make a smart stop to deny Cameron Jerome.
It still looked as though the Championship side were the more likely to make the breakthrough, however, with Earnshaw stinging Hart's hands for a second time with a driven shot from just inside the box.
Although Jerome again proved to be a menace on the break as he surged down the centre, holding off Luke Chambers, before flashing a shot wide of the post.
Birmingham suffered their second blow of the game as, before the half-time whistle, they had to make their second substitution.
Having already lost Stuart Parnaby to injury with former Forest loan signing Gary McSheffrey coming on, skipper Lee Carsley then had to be carried down the tunnel and replaced by Lee Bowyer.
Forest came even closer as Luke Chambers towered high on the edge of the six-yard box to connect powerfully with a Radoslaw Majewski corner, only for it to crash down off the underside of the bar and bounce away off the line.
The second half began with Hart at full-stretch again, this time to deny Cohen after good work from Blackstock had sent the midfielder scampering clean through on goal.
But the biggest missed opportunity was still to come, as Forest were awarded a penalty following a clumsy challenge on Paul Anderson by Gregory Vignal.
However, Earnshaw contrived to to sky the spot-kick over the bar in the 63rd minute.
The striker was a little unlucky not to score two minutes later when, having been sent through by Blackstock, his precise, low shot which was bound for the bottom corner was brilliantly saved by Hart.
As the action continued, Kevin Phillips was unfortunate to see an instant shot cannon back off the foot of the post for Birmingham.
The former England man was then left with his head in his hands again after a quick Birmingham break saw him played in for a one-on-one with Camp but the Forest keeper saved in outstanding fashion at his feet.