Reading were made to rue missed chances as Birmingham City took all three points with their only shot on target as Claudia Walker scored the decisive goal.
The Royals hit the crossbar twice in the opening 45 minutes but were frustrated by Birmingham’s organised defensive structure.
Tash Harding’s deflected shot was spilled onto the post moments before Walker scored the winner for the Blues in the 72nd minute to give them their first points of the season.
Reading came into the tie the form side having won their last three matches in all competitions, including a 4-0 victory over Charlton Athletic in midweek in the Continental Cup.
Meanwhile, Birmingham City had yet to pick up a point in their opening three matches to leave them just above the drop zone in the early chapter of the new FA Women’s Super League campaign.
The Royals had the first chance of the match when Lucy Whipp gave possession away cheaply in her own half which led to Brooke Chaplen striking an effort just over the bar from 20 yards out.
Reading keeper Grace Moloney collected a cross comfortably after Birmingham pressed forward for the first time, before Chaplen had another shot fly off target after a slick move in the build up with Jess Fishlock and Angharad James.
Amalie Eikeland showed her aerial presence as she brought the down ball neatly and teed up James who hit a half volley from distance which looped over the keeper but grazed the top of the crossbar.
The hosts continued to knock on the door as they searched for the opener as Fishlock forced a super save from Hampton who tipped the Welsh midfielder’s 30-yard strike over the bar after it had looked destined for the top right hand corner.
Chaplen threatened the Blues defence from the left wing on several occasions but Birmingham’s organised defence allowed them to frustrate the Royals throughout the half.
Amalie Eikeland looked to surge into the box from the left wing but was tripped just inches outside, leading to a free kick. Chaplen curled an effort from a tight angle which skimmed the top of the crossbar on its way over.
Kelly Chambers side were dominant in possession and territory throughout the opening 45 minutes but went into the break without a lead to show for it after Eikeland had a penalty appeal turned away just before James had another shot saved just before half-time.
The first big chance of the second half fell to Chaplen who worked some space past her marker but her shot kept rising as it flew over the bar.
Chambers made a double change just before the hour mark as the experienced pair of Fara Williams and Danielle Carter came on in place of Jeon and Eikeland.
Carter used her searing pace to get past a defender before she was fouled just as she was about to break into the penalty box, giving the Royals a set piece in a dangerous position.
Frustration began to creep into Reading’s game as they failed to create too many chances despite their dominant possession as Emma Mitchell sent a wild shot into the stands from distance.
Birmingham then had a huge let off as the ball somehow didn’t find its way into the back of the net after Tash Harding’s deflected shot was spilled and bounced off the inside of the post.
Just moments after hitting the post, the Royals fell behind as the Blues scored with their first shot on target.
Completely against the run of play, the visitors boke forward and Claudia Walker finished neatly past Moloney into the bottom corner after a cut back across the box.
Reading’s final change of the day saw Lauren Bruton replaced by Kristine Leine.
The hosts pushed hard to find a leveller in the final 10 minutes but the Blues stayed disciplined at the back to keep the Royals at bay.
Harding came closest to an equaliser when she stooped low to send a glancing header towards goal which fell just wide of the far post, but Birmingham held on to claim the win after a spirited defensive display.
Reading host Manchester City Women next Sunday in the WSL.
Reading Women: Moloney, Mitchell, Bartrip, James, Jeon, Fishlock, Eikeland, Bruton, Harding, Cooper, Chaplen
Subs: Leine, Williams, Nayler, Carter, Woodham, Skeels, Roberts, Harries
Birmingham City Women: Hamtpon, Mayling, Scot, Corsie, Green, Murray, Brougham, Walker, Whipp, Napier, Holloway
Subs: McCarron, Grant, Logan, Scofield, Toussaint, Lawley
Goals: Walker 72′
