By David Brenchley
A Rachel Williams brace, taking her tally to eight goals for the season, and Dunia Susi strike helped Blues increase their lead over the chasing pack to seven points.
It took the hosts just eight minutes to break the deadline when Mary Earps parried a Susi shot and the England international netted at the second attempt.
Half an hour later Williams doubled her side's advantage from the penalty spot after winger Karen Carney was felled by a combination of defender and goalkeeper.
Birmingham keeper Marie Hourihan kept their two goal cushion in tact with a string of fine saves, most notably from a close range Vicky Exley free kick and the result was put beyond doubt five minutes from time.
Williams grabbed her second of the game with a diving header from a Jo Potter free kick and Blues held on for the three points.
Second-placed Arsenal, who have two games in hand over the leaders, face bottom side Liverpool tomorrow and will be hoping to pull three points back in the race for the inaugural FA WSL title.
