But once Georgia Stanway struck a sweet low drive into the bottom corner, the floodgates opened for a Manchester City side who have now won 20 of their past 22 matches in all competitions, to move back up to third in the table and on course for European qualification.
Lucy Bronze and Lauren Hemp had hit the post in the first half but the home side ran away with it in the end as England winger Hemp, Australia defender Alanna Kennedy, fit-again winger Chloe Kelly and midfielder Laura Coombs all got on the scoresheets as the hard-working visitors eventually tired.
Stanway also got a second for herself, as Gareth Taylor’s team moved two points clear of rivals Manchester United ahead of the season’s final day on Sunday.
Manchester City (4-3-3): Roebuck 6; Bronze 7, Kennedy 7, Greenwood 7, Stokes 7 (Blakstad 7, 72); Stanway 8, Walsh 8 (Coombs 7, 72), Weir 7 (Angeldahl 6, 73); Raso 7 (Kelly 8, 45), Shaw 6 (White 6, 73), Hemp 8
Subs not used: Taieb (gk), Keating (gk), Mace
Birmingham City (5-2-3): Ramsey 6; Cowie 6, Scott 5, Lo Quinn 7, Lawley 6 (Lu Quinn 6, 60), Robertson 5; Finn 6, Murray 6; Sarri 5 (Whelan 5, 79), Pennock 6 (Ewens 5, 79), Smith 6 (Whipp 5, 69)
Subs not used: Hourihan (gk), Sandvej, Ryan-Doyle, Whelan, Jenner, Worsey
Referee: Elizabeth Simms (Halifax)