The pair sit 11th and 14th in the Championship table, respectively, and have a sizeable cushion between themselves and the relegation zone, but also have plenty of ground to make up on the top six.
Millwall's excellent form between November and mid-January saw them surge into playoff contention, but their momentum has recently given way to a poor patch.
Gary Rowett's side have won just once in their last seven games across all competitions - one in six in the league - and have slipped seven points off the playoff places.
The reverse fixture at St. Andrew's in November saw the points shared after a 1-1 draw, but Millwall need wins if they are to close the gap that is starting to open up.
Millwall's last outing saw them beaten 1-0 by Wigan Athletic, who are in the relegation zone, in a damaging defeat that was indicative of the low confidence in Rowett's squad at present.
Birmingham, on the other hand, are flying at the moment. The Blues find themselves unbeaten in 11 games across all competitions, with eight of those games coming in the Championship.
Rowett's former employers are now managed by Pep Clotet, who has instilled an attractive brand of football that has seen Birmingham score in 11 consecutive league games.
However, they have managed just one clean sheet in that run and have recorded only six across all competitions so far this term.
Their last outing - a six-goal thriller that ended in a 3-3 draw against Sheffield Wednesday - encapsulates their philosophy this season under the Spaniard.
Millwall Championship form: LDLDWL
Birmingham City Championship form: DWWWDD
Birmingham City form (all competitions): WWWWDD
Millwall manager Rowett has just a trio of players unavailable for selection ahead of Birmingham's visit to The Den.
Birmingham head coach Clotet has the same number of players missing as he prepares his squad for a trip to London.
Josh McEachran is out for the rest of the campaign after sustaining an ACL injury and is aiming to return in November.
Millwall possible starting lineup:
Bialkowski; Hutchinson, Pearce, Cooper; Romeo, Molumby, Woods, M. Wallace; J. Wallace, Bradshaw, Bodvarsson
Birmingham City possible starting lineup:
Camp; Colin, Roberts, Clarke-Salter, Pedersen; Bela, Gardner, Sunjic, Bellingham; Hogan, Jutkiewicz
We say: Millwall 0-1 Birmingham City
Millwall are strong at home, but Birmingham seem to have clicked under Clotet and could further extend their unbeaten run in London with a narrow victory.