Blues fall to defeat in snowy Cambridgeshire
Peterborough came from behind to condemn Town to defeat at Weston Homes Stadium.
James Norwood had struck inside six minutes but goals either side of half-time from Jonson Clarke-Harris and Joe Ward saw the hosts claim the points.
Paul Lambert made four changes to the side who beat Blackpool at the weekend.
Captain Luke Chambers, Stephen Ward and Norwood were all recalled while Teddy Bishop was named in the starting XI after serving his one-game suspension.
The latter two combined as Blues took an early lead. Bishop latched onto a loose defensive header before hooking the ball forward to Norwood, who let the ball bounce before firing a left-footed half-volley underneath Christy Pym in the Peterborough goal.
Flynn Downes was the first name in the referee's book just over five minutes later following a strong challenge on Dan Butler in the middle of the park.
Tomas Holy maintained Town's lead midway through the half with a fantastic recovering save, parrying Mo Eisa's low range strike after the ball had looped over the Czech 'keeper.
Eisa was the next to test Holy a few minutes later, his curling effort after cutting in from the left edge of the box going straight into the hands of Blues' number one.
Peterborough's left-sided forward continued to be their main outlet, curling over the bar as the game entered the final 10 minutes of the first half.
The hosts equalised a few moments later as Butler's pinpoint cross from the left wing was headed into the bottom corner by Clarke-Harris.
Peterborough took the lead early in the second half, Joe Ward scoring a free-kick from 25 yards.
Blues looked to respond as Troy Parrott dragged in three defenders before playing the ball to Andre Dozzell, who cut inside and saw his powerful drive blocked by Nathan Thompson.
Thompson came to his side's aid again a minute later, deflecting half-time substitute Gwion Edwards' shot harmlessly into the arms of Pym.
Posh could've grabbed a third on the hour mark but Sammie Szmodics and Eisa both couldn't get enough on Clarke-Harris' dangerous ball across the face of goal.
Town began to see more of the ball with the hosts looking dangerous on the counter, Szmodics drawing a smart save from Holy.
Dozzell picked up a booking with 15 minutes to go after pulling back the shirt of Idris Kanu.
Szmodics went close twice inside a minute as the hosts looked to put the game to bed. First, his deflected half-volley was parried by Holy before Town's keeper tipped his effort from the edge of the box around the post.
Blues came desperately close to grabbing a late equaliser from a corner as Parrott's strike was blocked on the line before Josh Harrop's follow-up was collected by Pym.
Peterborough: Pym, Butler, Thompson, Beevers (C), Kent, Eisa (Jones 74), Taylor, Clarke-Harris, Brown (Hamilton 74), Szmodics, Ward (Kanu 74).
Subs: Gyollai, Burrows, Blake-Tracy, Mason.
Town: Holy, Chambers (C), Woolfenden, McGuinness, Dozzell, Downes, Bishop (Sears 79), Judge (Harrop 64), Parrott, Norwood (Edwards 45).
Subs: Cornell, Kenlock, Matheson, Drinan.